Ex girlfriend getting with another guy?

So I went out with this girl for about 6 months, most serious relationship that we both have been in. I'm 19 and a freshman in college and she is 18 and just graduating high school. I commute and shes going to go to Hofstra (Huge party school, even though she doesn't party much). We brokeup in March and have been keeping in touch since but I started hanging out with a wrong crowd (people who smoke and sell mad weed) and got sucked into a huge drug scheme. I eventually realized that this is not who I am so I stopped dealing all that bud but by then it was too late she already foundout I was selling and her interest kind of faded because shes not into that. We still keep in touch though and I miss her alot, but her feelings aren't the same, Anyway today is June 19th, and this past weekend was her prom weekend. I coincidently went down the shore and saw her down there with a bunch of my friends and her friends. She barely talked to me and went off with one of my friends and they hooked up. I now realize that one kid is not my real friend for doing that. That's really ****** up to do, get with my ex? That I still have feelings for? Like wtf. I just don't know what to do. She bbmed me before apologizing for what she did...but why?