My bf & I have unprotected sex & he ejaculates inside me. It regularly burns, but I've noticed it doesn't at all when we use lube. Something wrong?

Friction - abrasions. The vaginal walls are not as tough as the skin on the outside of your body. The friction of intercouse can cause tiny abrasions in to walls of the vagina which would increase the sensitivity in those areas. The semenal fluid can be irritating to the abraded areas, causing the burning sensation. When you use a lubricant there is less friction, less abrasion, and less irritation. Nothing is wrong.
Mabye. Burning after sex that is prevented by lubrication probably means that you are not making enough lubricating fluid yourself and the friction of sex is irritating your vaginal tissue. Causes could be not enough effective foreplay before insertion, or inadequate response due to medications or post menopausal state.