<human>: Can you give me the gist of the text in a nutshell?

Me and my ex have a very complicated relationship. Dated almost 2 years through college (same college), then broke up but it was more of a break. However, we were talking during this break, and I hooked up with a couple chicks without telling her. Once again, we were talking but not dating. 

Anyways, we start talking again for real and are "exclusive" yet not actually dating. But, shortly after she finds out I hooked up with her sorority sister during our break (she thinks it was multiple) and goes crazy, and says were over.

She told me that night she madeout with a dude. I was very mad but got over it because I understood her frustration and things started progressing again. 

Just today though (a month later) I found out she actually had sex with the dude. I ask her and she says yes she did, and she didn't tell me because she knew I would've been so mad I would've never talked to her again.

And I understand that she is right. I would've been furious. However, is this something I should get over or is it a huge red flag? I'm scared she has done this before.
<bot>: GF breaks up and one night stand same night, says it was makeout to not hurt my feelings and to not lose me, I find out and now I don't know what to do