Do I Trust her? READ STORY GIVE ADVICE PLS

Ok so I've been sleeping with an ex lately, things ended rather poorly about a year ago. She used to be so innocent, but now I hear that she sleeps around like it's nothing. (Which makes me just feel used) I spilt my heart out to her one night only to wake up to a snapchat on my phone of her hooking up with some random guy at a party. She later brushed this off a just "being drunk." The next party we both attend goes like this; she gets driven there by an "old friend" (an old hook up) and they are uncomfortably close all night, she pretends to be clingy to me for a little bit until the weirdest thing happens. The other guy she was with pulls me aside, tells me that we are boys and that we are all such great friends (while he's got his arms around to two of us), he then walks away - slaps her ass, and she proceeds to tell me that "we're really good friends" ad we're "boys". They later cabbed home together and stayed at her place. She has told me that they never have done anything but this seems to happen a lot. Any advice would be great guys this is really tough