Friend(18F) became distant and ghosted me(19M) but I don’t know why

There’s this girl(18) that I(19) have known for 2 years and I really like. She’s part of my friend group so we see each other fairly often when we go to parties since we don’t see each other in school anymore.

Two weeks ago, I decided to start talking to her in a more flirty way. I am not good at this, but I was not going overboard. It was just compliments about her personality and looks. We were talking well and she invited me to go to the club with her. I told her that I couldn’t since I had plans with the same friend group about going to a weekend trip and I invited her to go with me. Since it was four guys and every other guy except me brought their girl, I didn’t want to be alone so I invited her.

Things were going really well on Friday since I drove alone with her for 2 hours. We were talking good and when we got there we had a barbeque where we were drinking and having fun. I was talking to her a little less since all her friends are there too, but still somewhat good. She even left to talk to one of her friends in a room and I overheard them talking about me. We slept in the same room, but not together, and we were talking for a while before going to sleep.

The problem starts Saturday. We woke up and talked and ate breakfast. But around midday, she just stopped talking to me alone. She wouldn’t talk to me unless we were with friends, and even then it was to me directly. This continued Sunday.

When we came back, she would not talk to me in the car but she blamed it on being tired. However, when we met up with some friends after we got to our city, she wasn’t tired anymore. I tried texting her after but she never responded and even unfollowed me on Instagram.

What did I do that made her act this way?

I know I should just let her go but I have really liked her for a long time and it’s hard to do it considering I’m going to see her again. What should I do?

TL;DR: I went on a weekend “date” with a friend of two years and now she ghosted me after us being good for half the weekend and then becoming distant for I don’t know what reason for the other half.