Is this cheating?

My girlfriend and I have been together, well talking, since the middle of October.

Last night i got bored and went through her dms (because she usually sends funny videos to her friends so I wanted to cure my boredom.) and what I found broke me.

She texted her friend in November (like the third?) and said that she had sex with someone, note this is the day before I was supposed to hang out with her. She sent a picture of a hickey the guy gave her and said “I felt like I cheated.” It broke me.

I confronted her about it and her response was “ I didn’t know we were going to be serious then, we weren’t actually dating” but what hurts me is that before then she told me how she’s only ever been cheated on and how hurt she’d be if I cheated.

So I’m just confused on what to do. I don’t know if I should stay or if this is even cheating? I need help.