Help Me!!!

My boyfriend and I have been having sex for two years.. He's the only person I've ever been with in my life (I'm 19) I out my virginity to him. He's pretty large.. It's never caused me pain up until now, it feels like he's just too big for me. Why is it causing me pain all of a sudden? I don't have any problems getting wet and we use lube and non latex condoms. I'm concerned.. Please help!!