<human>: Can you condense the text into a few key points?

So me (22 M) and my ex (22 F) met through mutual friends and dated for a little over nine months. It got to be a pretty serious-ish relationship and then things ended up not working out. We didn't blame each other for what happened, it just didn't work. No big deal. I've been in a relationship since then and she's got a boyfriend now too. 

We would talk occasionally--briefly, every couple of weeks--and then out of nowhere she texts me saying that we can't be friends and to not talk to her. Naturally, I called to try to find out what happened and her boyfriend picked up, telling me that I needed to leave old feelings for her behind. I calmly explained to him that there was a misunderstanding, I didn't have feelings for her, and we settled things. I talked to her a week later and she gave me an explanation (it was a drastic reaction to a fight she had with her boyfriend) but the main point is we got it worked out. 

We had a few more conversations where we laughed and got along like normal. A few days after that, she gradually became distant. She wouldn't respond to texts that she read and barely spoke to me when I saw her in person. She doesn't seem to be so distant from our mutual friends, although they have commented on the fact that they think something is going on with her. Knowing how she tends to operate, I resolved not to pry and to let whatever is going on be her problem, not mine. 

My concern is that one of our mutual friends wants to get a small group together for a beach trip later this summer and she'll be going along. I don't want to deal with passive aggressive hositility the whole time and I don't really know how to properly address this. Help?
<bot>: My ex started refusing to talk to me out of nowhere and I don't know how to deal with her hostility because of our mutual friends.