abusive relationship

After finding out my ex best friend is now friends with my other ex best friend after them hating each other really upset me .. I have done so much for one particular friend, and she just dropped me like I didn't mean shit. The 3rd time she dropped me and then came back and said sorry, I took her back all those times.
Now to find out she is friends with a horrible horrible person I was friends with for a year.
Anyways, when me and my ex girlfriend broke up, she befriended my ex best friend after they hated each other. Slept at her house and hung out.
When I found out that these girls were friends last night on fb, i told my girlfriend as we got back togather a year ago.. 
I told her how I felt and upset I was, and it blew up massively, it gone from me saying how upset I was to her calling me a fuck head, and amongst other words!
I feel like, my girlfriend who is friends with my ex bestfriend, she's two faced.
I get the gut feeling she lies to me when she hangs out with her.. 
she saids she doesn't talk to her , but I feel like she does. 
I don't know .. I'm so upset about these ex friends being friends when they hated each other and the only reason my first ex friend ditched me was cause I was friends wit the other one.. stupid really..