first, i want to say that you are validated in how you feel. your girlfriend weaponized her past feelings for this person as a way to make you jealous, which is not okay. no matter how long ago it was. from what you've told us, it doesn't sound like your girlfriend really understands the mental gymnastics she has put you through in regards to this situation. it seems like she wanted you to be jealous, and then once you two started dating, she tried to foster a friendship between the two of you (let me know if i misinterpreted that part of your story). that right there is a giant red flag to me, because in no way should she expect you to be friends or even cordial after telling you that she had past feelings for this guy. if it were her, i doubt she would want to be friends with an ex or someone you had feelings for.
it seems like you also told her that you were uncomfortable with them speaking again, but she did it anyway. and in my view, you are not overreacting by being upset about her liking a shirtless picture on his facebook. it's inappropriate and a clear crossing of a boundary that should have been set, like she said, if they were going to try and be friends again.
the only thing you can really do here is have a chat with her. i wouldn't come at it from an angle of accusing her of anything, but just tell her how you really feel. try to explain it to her in a way that will allow her to see it from your point of view. i feel like if she respects your relationship, she will take what you say into account and most likely stop talking to this person.
