A girl who seemed eager to meet me on the first day now seems to act uncomfortably around me. What am I doing to cause it?

Hello. Thank you to anyone taking the time to read.

TL;DR – New girl at work seemed eager to meet me on first day but since then seems increasingly uncomfortable around me without me being able to trace it to anything I might have said or done.

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Two weeks ago a girl (19F) arrived to make an internship for two months at the place where I (31M) work. She is from another country but with similar language so that each can speak their own language and understand each other. She was assigned to collaborate with a colleague of mine who works on the same floor as I do and with whom I interact frequently. For the time being she is not having to work 1 on 1 with me, and it’s likely that for the entirety of the duration of the internship she won’t have to. Any task she might have to do involving me will very likely involve the presence of my colleague as well.

So, in the first day she arrived, my colleague and I were assigned to show her around the place. She seemed comfortable and easy going and talkative with both of us. At some point during the conversation, she asked our ages. My colleague is 24M and I’m 31M, her response was that I really didn’t seem to be that old and that I appear younger. Anyway, it was all a normal interaction with each of us asking normal questions to get to know each other better. By the end of the “tour”, my colleague had to stay behind to talk to someone else and I went along just to show her the exit. At this moment when it was just the two of us, she said that “It was nice to have met me” and stretched her hand to shake hands (we were both using masks). She also said that “I was cool” and she even asked “we will be working together in the future, right?” since she already knew she would work mostly with my colleague.

Now mind you, I really did not say or do anything special or suggestive to her during this tour, I did not compliment her looks or attire, I did not make any kind of advances, so these words she spoke at the moment she left for the exit were of her own volition and I wasn’t even expecting it, and could only presume that she is kind by nature.

But I have to admit, I found her very attractive, so the fact that she appeared to suggest that she wanted to spend more time with me in the future made me start wondering a bit, given that I’m currently single and have been single for a while now, and I stayed with that lingering feeling inside me “I wish I could know her better, what she’s like”.

Anyway, since then she has been working all day alone with my colleague (who has a girlfriend already and is a serious person). I only interact with them both once or twice a day for brief periods of less than 5 minutes. I know she is talkative and easy going and even playful with him. Now, this is perfectly normal, considering he is closer to her age, so they have more in common to talk about. But the strange is that since that first day and progressively over time, it seems she is nowhere near easy going around me anymore. When I step into their place or when they come to mine (mostly to exchange info or material) for these brief periods, I mostly speak to my colleague and address him, with some remarks to her on the side to acknowledge her presence. Usually, the remarks to her are normal questions/statements that just have to do with the work at hand. I’m not a joking person in general.

But some signs of discomfort I’ve noticed when in my presence during these brief exchanges include a) grabbing her phone as I enter, distancing herself from the conversation between me and my colleague b) crossing arms (some say this is negative body language – but it just happened once so hard to extrapolate) c) When I address her, she replies politely and smiles but never delves or furthers the conversation or make follow-up questions, just says enough and not more d) avoids eye contact when not talking directly to me e) staying silent and distanced / backed off unless directly addressed.

I’ve been trying to figure out if there’s anything I have done wrong in my behavior to have caused this, and I really can’t think of anything explicitly that I might have done. I honestly tried to inhibit myself from giving any signs of interest or attraction, and my exchanges with her have been superficial and not delved into personal matters. Now, I have no idea if she has a boyfriend, and I haven’t asked. Maybe if she has one, the fact that I’m single and older might scare her into thinking I’ll be trying to make an advance at her and cause an awkward situation, or maybe she is afraid since I’m single, I might be turning into a creep over her, but back in the first day, after she knew my age she didn’t seem minimally bothered by it (as confirmed by the way she later said goodbye).

Obviously given the age difference and the fact that she is to stay 2 months and be gone, I’m not going to make any advance, but I’m just intrigued more than anything else. How can I troubleshoot this?