Every time after sex my vagina swells a little and hurts, I have seen the doc 2x and he says I'm good. What could the problem be?

Yeast? Low estrogen? I would be suspicious of a yeast infection, which can be hard to diagnose even on careful exam. Discuss w/ your doctor, or maybe worth trying an over-the-counter vaginal yeast treatment. Low estrogen might do it, but unlikely at your age, assuming normal menstruation. Continue to work with your doc or maybe get a second opinion. It should be possible to diagnose and treat this.