My girlfriend calls and meets often a guy that she dated 3 years ago?

My girlfriend has this guy friend that she calls often every week and meets him every other week. They dated 2 years ago and she didn’t tell me about until I asked her. She told me that they had kissed each other but never went beyond that. Ive told her that it bothers me a little and she tells me not to worry, but it does. She often talks to him and then they meet every other week for lunch and stuff like that. A couple of times she told me she was going out, and then I found out she went out to meet him without telling me. So I was thinking whats your guys opinion on this matter? Is it ok for your girlfriend to meet her ex and talk to him a lot? Cuz its bothering me and although I’ve said that to her she still calls him often and keeps seeing him