You may want to see your doctor. If not, definitely avoid inserting anything into your vagina until it clears up (no sex, no tampons).
Did you use a condom when you had sex? If so, you may have a sensitivity to the condom (the latex, the lubricant). You may want to try a different brand...also be sure to use lubricant...what you describe could be from a lack of lubrication.
It's weird that it was 3 weeks ago...but pain is different for all of us. It sounds like you are just sore from having sex. Maybe just wait, if it keeps bugging you go see your doctor