my female friend

When I transferred schools I made loads of friends and one in particular was special. She lived 2 blocks from my house and we hung out loads, watching movies and baking sometimes. We were just friends but loads of people in my school would tease us about how a good couple we made. I would deny it every time somebody asked me if I was going out with her. One time this girl asked me when we were walking home and I kinda yelled "NO" which disappointed her. So the prom was coming up and I wanted to ask her to go. The problem is that yesterday her friend came up to me and told me she was making out with some other guy in the bushes...and yes it was true. I'm really worried, she is such a cheerful and happy girl and her new boy friend takes drugs and drinks a lot. I'm really worried about her changing...and now that she has a boyfriend I feel like a retard because I didnt have the balls to ask her out. PLEASE someone help me out!!!!!!!!!!!!