Avoiding pain?

So recently whenever me and my bf are having sex, it all feels so good until he actually goes inside me. It's not a matter of wetness because we've tried lubes. He keeps saying we just shouldn't do penetration but I get to a point where my body is just aching for it and I want it so bad. But then it hurts. Almost in a way that I'm being stretched too far. It's in all positions too. 
We've never had this issue until the past two months, we've both been cleared as STI free, and he's the only one I've ever had sex with. 
So what are some recommendations for avoiding pain or ideas as to what's causing it?