ah man thats pretty shit, if it does happen that you get finished, then follow the advice below, if she gives you another chance, then just remember its easier to forgive and forget then it is to argue .....dont start sending angry texts to her, just let her say her piece...
EDIT: I notice you said she was 'out' last night, which i think you mean she was on the piss. I would let her have her time to cool off, then let her come back to you to speak to you.
ADVICE
to be honest, your young, your at uni, (still at dundee i imagine??) so there are plenty more fish in the sea and all that.
I would try not to linger on it too long, maybe go out and get a bit o sleazin done to make you realise this girl wasnt everything....then you can hope for a more stable relationship after this . You never know, this same girl might realise what a mistake she has made.