Would mean a lot to me if someone could please spare a few mins and help me with this I don't know what to do?

Me and my boyfriend are together a year now and had just moved in together. However at Xmas when we split up hengot in Contact with this girl he used to be seeing 2years ago and started meeting up with her again but swore they were only friends and he was never alone with her. Then when we got back I presume he stayed in contact by text but I know he didn't meet her and he even deleted her from his FB for me but when we split up 2 months ago he started meeting up with her again and he brought her to his match and she stayed over at his mates house. Now were back together and have moved in but i dont know if she knows this. I do believe he hasn't been in contact with her since I moved in but last night his sister in law put up photos of me and him togther on our bday and this girl left a comment but by the time we say it it was deleted, he seemed very worried when he seen on his phone she left a comment but then when we seen it wasn't there I asked him to ask her what it was and e refused. Am I being made a fool out if here? Should I let it drop or find out what it said ? Please help