Current guys Ex's

I know people have ex's. I have an ex. But for some reason, I get upset when the guy I'm dating talks about his ex! I don't express it to him, I just get sad and my stomach drops (while acting normal). 
He doesn't talk about them often. He's had two serious relationships. 
Just recently I was cleaning and found her sock under the bed, I'm sure it was there for awhile but still it made me remember that he has had sex with her probably in the same way we do now. There are also still plastic confetti hearts scattered around behind stuff (found when cleaning) and a few hanging from the ceiling. It was for valentines 2016 apparently. And today, he said he found a bobby pin and I said it wasn't mine and he said it was probably my ex's. I know it's irrational and I know that it's not a big deal because he's with me now. But it makes me feel sad. 
Does anyone else get like this?