Pain During Sex

I've recently started sleeping with a new guy and have found that it's really painful to have sex with him. We've tried drawing out foreplay to make sure I'm turned on enough, and we've tried lube, but neither seems to work. Could it possibly be that his penis is too big for my vagina? Furthermore, the one other guy I've had sex with wasn't painful in the slightest. What's going on, and how can I alleviate the pain?