(M25) Need advice – Getting mixed signals from a girl

Hi, everyone,
There’s a girl that I’ve been interested in for a while, and I believe that she feels the same, but I’ve been getting a lot of mixed signals from her. Any help would really be appreciated.
For starters, I’m a 25 year-old male and have never had a girlfriend.
Back in January of this year, I started doing volunteer work at a post office to get myself more active and involved.
I was feeling tired that day, but something in my heart told me that today was special.
I ended up meeting a girl on that day. Let’s call her Jane, for the sake of the story.
While waiting for the office to open up for business, I made casual conversation with her, asking her basic questions like where she was from, and what not. She was a few years younger than me, but carried herself well.
As the conversation continued, We ended up having a lot in common! She was also smart, funny, witty, and pretty. We seemed to have some very similar goals as well.
The more common ground we discovered, the more I noticed her eyes light up with excitement. I also noticed her leaning in to listen to me intently, and playing with her hair. All of the signs were there.
Still, I made no attempts at flirting with her, nor did I show interest.
I didn’t ask for her number, for a date, nor did I compliment her. Just treated her like a normal person. This was mostly due to it being a working environment, and me wanting to remain professional.
I wanted to finish our conversation, but the post office opened, and it was time to get to work.
After putting in quite a few hours, I left for the day.
I thought about her a lot after that, and considered coming back.
“Don’t go back there JUST for her”, I told myself. “That’s weird and obsessive.”
Days turned into weeks and weeks turned into months. I hesitated, and grew busy with work and school.
“Forget about it, she’s probably long since moved on, and probably doesn’t even work there, anymore”, I thought to myself.
I ended up returning in May.
When I walked in, I was shocked to find that she she was still there. I was speaking to someone else, but could see her sitting at a desk, logging things into a computer. I could see her from the corner of my eye. She glanced at me with bewilderment, then frowned.
We didn’t acknowledge each other nor speak.
I decided to start volunteering again. If she just happened to be there, then so be it.
The next day, I started avoiding her like Pacman, afraid of an akward confrontation after my long hiatus.
I eventually decided to try saying hi. While lifting some boxes, I lifted my hand to wave, but she turned away, wincing, as if someone had punched her in the stomach.
This would continue to happen whenever I passed her.
Periodically, I would catch her turning around to glance at me, when she thought that I wasn’t looking.
Sometimes, when I was speaking to others, I could see her smiling to herself, warmly, or observing the conversation from a distance.
Still, she avoided eye contact and speaking to me, unless necessary, like something work related.
Personal conversations were never a thing, only small snippets and short responses.
I got along with everyone there, but she seemed to be the only one that didn’t really speak to me; a complete 180 from before.
One day, the staff decided to invite me to a dinner outing, which I happily accepted.
Only six of us showed up, and she wasnt there. As we were leaving the restaraunt, I noticed her show up to the restaraunt with a guy.
The guy greeted all of the other guys with hive fives and handshakes.
I decided to say hi to him.
“What’s up?” I nodded.
He scowled, looking me up and down as if to size me up. (Even though he was about 2 inches shorter than me. I’m 5’9)
“Sup…” He replied warily, before the two moved on.
Confused, I asked what just happened.
“Her boyfriend showed up”, one of them mentioned.
I was shocked. A boyfriend? Had I read the signs wrong? Was I just being lead on this entire time? How would this guy even know who I was? I was bummed out.
It just wasn’t adding up. If this was truly her boyfriend, then why was she glancing at me from afar?
On top of that, if she only saw me as a friend/aquaintance, there’d be no reason to act so akward around me.
I wondered if this was a friends with benefits relationship?
Things began to change over the next few weeks. I never made any moves on her, but observed her behavior.
She slowly started to warm up to me. First with small sentences, then by shyly waving and smiling at me, but no direct eye contact. When she would wave, it was almost as if she was looking at something behind me.
Whenever she spoke to me, she would look at somebody else, or at the wall/floor.
This is all baring in mind that she never flirts with me, or stares lustfully. We don’t even speak for the most part.
She had no problem speaking to any of her other male colleagues, but was silent and odd around me.
There was even an instance where I was speaking to someone else, and she passed between us. She gave me the warmest smile ever and waved, before moving on. She still didn’t speak, though. I smiled and waved back, greeting her with good morning.
Ordinarly, I would just call this shyness. However, she was nothing like this during our initial meeting.
Things were getting confusing at this point, and driving me crazy.
At this point, I had to do something. But how could I speak to someone who wouldn’t speak to me?
On top of that, this was still a professional working environment, so I had to tread carefully.
I pulled one of the guys that she worked with aside and we talked in private.
I mentioned that I noticed her looking at me, and could tell that she liked me, based on her behavior, but needed to know if she had a boyfriend.
“Yes,” he responded.
“Then why is she looking at me like that?” I questioned.
He shrugged his shoulders and said that he couldn’t tell me.
He explained thats simply the way she is and she has a quirky personality. Even with his heavily introverted, quirky personality, he could never understand her.
I calmly acknowledged and thanked him, before going back in. I’m not the type to pursue someone in a relationship.
At the end of the day when I left, I was heart broken and frustrated. I began to tear up.
Was it all just in my head? Was she just using me for an ego boost? I didn’t know what was going on. I felt used and tossed to the side. I rarely develop feelings for a girl, but when it happens, it’s almost as if it always turns out to be some kind of joke.
The following week when I returned, something happened.
She was there, but didn’t look or speak to me and vice versa.
There was an uncomfortable silence in the air. Someone had told her. Maybe I was overthinking it, but I had a strong hunch that the secret was out.
The final verdict seems to be:
We like each other, and both know it, and that I know that she has a boyfriend.
The question is, what’s going on?
No matter who I asked, or how much I played the scenario back in my head, none of it was adding up. I could only think of two possible reasons:
1) she’s unfaithful. Plain and simple.
2) She may have a boyfriend, but she’s not happy with her relationship. It’s possible that she still had feelings for me, but settled on the current guy. When I returned, she was shocked that I was still around. If this is the case, I feel if she harbors feelings for me, she needs to discuss any issues with her partner, first. If he doesn’t comply, she needs to properly end it, before pursuing anything.
for what it’s worth, I was told that they started dating in April, only a month before I returned.
Any advice from anyone (preferably female), that’s been in a situation like this, would be much appreciated. Thanks in advance!