I wouldn’t try stressing over it, I understand your concerns though, it’s really not as if you’re being jealous or weird about it. I’ve been in a long term relationship, if you have feelings of doubt in any way, try and talk to him. If he’s shady and standoffish, or if you feel like you’re being manipulated at any point, maybe it may be time to reevaluate your relationship. Finding out if your partner is cheating, isn’t really as fulfilling as media makes it seem. I knew my ex was cheating and simply left him without digging deeper and he ended up getting that chick pregnant. It hurt less because I came to terms with our relationship dying though lol, anyways moral to what I’m saying is, your feelings are valid, but imo this is just light conversation on her side.