Most Helpful Guy
It's normal. When one goes through a break up and they see their ex with someone else, it's kinda hard for them to imagine their ex being happy with someone else. They just want to feel like "Yeah I'm the best they'll ever have", when in reality that's rarely ever the case.
No one likes feeling replaced, even if they're the ones who ended things.
Most Helpful Girl
It's natural. It's just a weird thing people do. Your not the only one. It's probably just weird seeing him with someone else.