I got that too about 5 months ago. I ignored it and the pain went away. After that I started getting really fatigued and i thought I was sick, but the syptoms didn't really seem like any minor bug. Then it came back way worse. It was so bad that the pain kept me from sleeping, or even laying down all night. I had to go to the bathroom every couple minutes, and it hurt! even though my bladder was virtually empty! I went to my gynocologist and she gave me pills for a bladder infection. This can happen from alot of things, but since it was after sex, ether his penis wasn't very clean or he fingered you and his finger weren't very clean.
You need to see a doctor. It could be as simple as a UTI but it could be more than that.