Why does it hurt so bad??!! HELP.

So i recently started dating this new guy. And we had sex for the first time. It was all so great except for the fact that in certain positions it was just PAINFUL. I didn’t wanna say anything at first but it literally got to the point where i couldn’t take it, so i told him to go slower/gentle. I’m not sure why this is happening, never had an issue before him. There’s enough oral for me to be wet enough, it could be maybe his size??? He’s a lot bigger (thickness wise) than anything I’ve ever had. Could that be it? NEED OPINIONS PLS, THANK YOU IN ADVANCE😊😊