I thought she was my mate :'( ?

You're looking into it to deep. If shes loud and stuff then perhaps thats just her way of having fun! Plus i don't think she meant it personal if she was doing it with everyone else. You shouldn't call her a tart, shes just more outgoing than you, shes not exactly sleeping with them is she? You obviously like this guy a lot because your blinded by it, ignoring logic and reason. If she's your friend then why don't you talk to her about it. Confront her. Odds are I reckon that you were just a bit shocked.