I can't get pregnant

I lost my virginity to the same guy I am with. I'm 18 and even though yeah I'm young but I think I can't get pregnant. We never use any sort of protection. We are not trying to get pregnant but I think something is wrong with me. We have sex a lot and then we decide to slow it down cause of work and school. He is 23 and he has a son. What could be wrong with me. ???