me and this girl have been very close for about a year and a half and shes always been there for me even though we argue and disagree a lot, we have never really been an official couple but we act like we are, recently we stopped talking for a few weeks and in that period of time she dated a kid for a week and lost her virginity to him. i didn't know about this until my best friend told me today that she told him about that and said not to tell me. i've been being an dick to her and her friends since this morning when i found out, should i get over this and continue on or should i just forget about her for trying to go behind my back?
Since there was no committed relationship and you two didn't seem to be hitting it off very well (arguing all the time), and you were not even on speaking terms, there was no reason why she could not hook up with someone else. There really isn't any "going behind your back" about it. As you say, you weren't really an official couple.
I know this is a hard thing for you, and it is understandable that you are upset, but you need to move on because she has. It's a good time to stop seeing her...trying to start things up again aren't going to settle your differences and now you have this little thing in your history with her.
There are other girls out there that you will get a long much better with, and won't put you in this kind of situation. Forgive, forget and move on and best of luck to you.
"or should i just forget about her for trying to go behind my back?"
Go behind your back ?!? You weren't dating her. She doesn't owe you anything. You're just a friend. If you want to be her boyfriend, then ask her out. But don't get pissed at her because you think she "cheated" on you. She didn't - YOU'RE NOT A COUPLE.