Why do I do this to my self and get upset

I've been with my current boyfriend for a month and a half. He tells me he loves me and I believe him. We are both very happy. But I still can't help but stalk all his ex girlfriends and get upset by it. I know it will upset me but I can't stop. One of his ex's now has a child and I'm paranoid it looks like him. He told me he's never got a girl pregnant (that he knows of) but I just can't help it coming up with crazy scenarios in my head. He won't be Facebook official with me but he was with his 2 exes. I know I'm being stupid and I have a lot of insecurities but is anyone else like me and always looking, thinking, that things are too perfect and something has to go wrong?! ☹️