is my girlfriend cheating??

So, a few months ago my gf and i worked the same job. We had a coworker who we would hang out with (go to her house to chill and drink basically). I had a weird feeling about the co worker for a while but then it happened. I was in the living room at co workers house when i walk in and see them kissing each other (co worker and my gf). My heart instantly shattered i didn’t know what to think or do. We talked about it. It was just a “drunk thing” and one minute everything was fine, next minute co worker is kissing her. Okay. Afterwards, my gf continues to talk/text co worker on a daily basis. I mean every day. I eventually said that I couldn’t handle that. Her talking to co worker was just too much like the kissing was bad but you continuing to talk to them on a daily basis after that is not okay. She said she would stop. Fine. Well, I pretty much put it all behind me until my gf started talking to coworker every day AGAIN. Only reason i even knew is because I noticed her being weird with her phone (hiding it from me, never being on messenger/text when i was in eye view) i notice the small things. So i realize shes talking to co worker again. I ask her what’s going on and she says it only work related (they are both managers they do have to communicate i get that). However, I was on fb and seen a post and saw that my gf had comment tagged co worker in it (just a cute cat video but still). I wasn’t really sure what to do so i started to kind of be on the lookout, trying to see if i noticed anything else off. I know she still talks daily to co worker. If not daily, then every other day. I’m not really sure what to do or even say at this point. I hate bringing it up because it was a pretty minor thing but i made it clear that it hurt me. Do i just let it go? Bring it up? Talk to her about it? I feel like we’ve talked about it so much but seem to not really get anywhere. Usually just get apologizing telling me she’s sorry for screwing up and blah blah. Is this even considered cheating? I’m just so in love and don’t want to give up on that very easily but i’m starting to not feel loved back because of this. Any sort of advice would help! I just really need an outside opinion i guess. If i need to elaborate please tell me and I will do my best! Thank you! ~