I am going through a bit of a rough patch with my SO. [32F, 47M]. He's been away for quite a while on business but we're used to that. Communication has been a little more sparse than usual, but he says he's just been super busy and tired. I got onto a phone he left behind to look up a number and saw that he had messages so I checked to see who they were from so I could tell him who was trying to get ahold of him. I saw that one was a woman who was in a group we used to be a part of. She caused some huge drama and blamed it on me, called me awful names, and pretty much broke up the group. So I opened the message. We can casually get on each other's phones but blatantly opening each other's messages just isn't something we do, it's far too invasive. I shouldn't have done it but I was too curious. She was talking about how much he must miss her and he said he misses everybody. She went on to make a couple of very flirty comments and he responded in a mildly flirty way as well. I don't mind mild flirting, everyone does it and it's just part of both of our personalities. My problem is that he would flirt with HER, someone who had treated me so poorly in the past. That mixed with our lack of real communication lately has me feeling a bit uneasy. Any advice on how to approach this situation would be greatly appreciated.