What in the world is going on with my bf??...?

My bf and I are both 18 and have been together for 9 months. We have had our ups and downs, but last night was definitely a major down. I was babysitting these 2 kids for about 4 hours last night when my bf texts me and says , "Umm who is in the house with you"..i said, "me and the kids why do you ask"....and he replied, "tell me the truth im not stupid i just drove by and I saw someone else in there"...now I had thought I'd seen his car drive by so I knew he saw the house (he lives near where i babysit so i wasnt completely freaked out but just thought it was rude of him to go out of his way to stalk me)...anyways, I called him (he didnt pick up)..soooo i left him a voicemail saying something like, "I would never call you stupid but youre definitely not right about this, it's me and the 2 kids, and no one else 1)why would i sneak anyone over while im babysitting and 2)if i ever was stupid enough to sneak someone over you should trust me that it would be you". Well, my bf continued to ignore me for ab a half hour. After about a half hour he answered and asked, "why do u keep calling me"...and I told him that I love him and that i need him to trust me and apologize to me for accusing me of something i didnt do. He refused to apologize bc he "knows what he saw". Then after babysitting I went to see him for a few minutes, and he still refused to apologize. I broke down in tears when he still wouldnt believe me, even though he wasn't mad at me, he still was acting like he "knewwww" exactly what he saw...and it was driving me insane bc nothing actually happened!! Well, once i was crying he comforted me and was all loving and sweet and I forgot about being mad. I'm really hurt that he doesn't believe me about this. Was he just making up that he saw someone else? Or is he really that paranoid of me? I've never done anything to make him distrust me that much.