Is it normal i have suspicions?

My boyfriend and I have been together for 2 years now. It's been really great! We're long distance but we make it work. We also trust each other. We're both the same age, 19. And we have a lot in common.
Around this time last year, my boyfriend and I decided we would have sex. We were both virgins. He was really good to me, I was in a lot of pain, but he knew exactly how to comfort me and he was really gentle. (I'm a very petite girl, and have a shallow vagina, and he's a very tall guy, with a really big penis.) Breaking my hymen, he was very careful not to hurt me too much. He knew I was very small so it hurt me a lot more than it would for most girls.
During sex, I just couldn't stop thinking about how good he was. It was odd, cause I had no idea what I was doing, cause I was a virgin. I expected the same from him, cause he claimed to be one too. But he was really good, knew exactly what to do. He seemed experienced.
I know he's had past relationships with other girls.. 6 girls to be exact and 7 including me. But he always says I'm his first real girlfriend and that nothing happened with any of them, they didn't last long. He's talked about one girl he's been with that liked having her hair pulled while making out. I thought this was odd, because any girl who knows they like their hair being pulled would have to have had it done before and normally that happens during sex...
I started to think that maybe he's had sex with someone before me, that he only said he was a virgin because I am. But every time I ask him if he's experienced and how he knows just how to make me feel good. He shrugs and says "I don't know, I just go with the flow. I never had a partner besides you."
Is it normal I have my suspicions about him? Or am I just over thinking?