Hi Lil bad,
First, I've asked the mod to move this to the Dating section... you'll probably get more responses there.
To be clear... she told you she wanted you to be her first, but while you were on a break, she had sex with someone else? *
Well, if you weren't together at the time, while you may have "hoped" to be her first, many times in life things don't turn out the way we hope. * If you were on a break, I don't see that she did anything wrong. *
As I see it, she's been honest with you. *If you have feelings for her, hopefully you can get past this. *However, if you can't, then *you should probably just move on now... being angry with each other isn't going to accomplish much.
Take care.