Girlfriend cheating, what do i do?

We are both 16 and go to different schools and they guy that she is seeing behind my back goes to her school so i can't be there to judge but I found out from her best friend who is also my friend. I don't want to believe it but I know something is wrong because she has been acting so strange lately. We used to have time to cuddle, hug eachother, kiss ect and she doesn't call near as often, but now I can see quilt when I look at her and I asked if she wanted to talk about it but she claims it is because she is still shy around me, but she wasn't as shy as she is acting now. I want to talk to her about it without acting like I am accusing her but I am not sure how I would. I am also thinking about doing something special for her like buy her a heart necklace and write a note explaining how important she is to me and see if she is happy or that quilt is there. I really want to make things right