Last night while we were having sex, I experienced pains when my boyfriend was fully inside. It was almost like a cramp, but more painful and to the left of my uterus. We stopped before we finished but the pain stayed. It felt like severe gas for the next hour but didn't fully stop for another few hours. Going to the bathroom was not the solution, it didn't help at all. What was this caused by?

You need to go see a doctor, because I have no idea what it is without examining you.

It could be something as harmless as gas, or it could be something more severe like a cyst. Best to get an OB/GYN exam to be sure.