Hurting during sex

I'm a virgin well I was and me and my boyfriend just recently started having sex but the thing it when he's all the way in I get the worse cramping feeling right in my tail bone and it hurts like a bitch and then the way his penis is shaped it's like thick at the top and slightly smaller near the base so idk if that has anything to do with it I just need help. Is there something wrong with me? And what is that cramping feeling caused by ?