Why does your vagina lip become swollen after having sex?

See a gynecologist: what matters most is the time course of your symptoms. If this is something that just happened in the last few hours, it is unlikely to be a yeast infection or a bacterial infection. More likely this could be a result of just vigorous intercourse or an allergic reaction. Regardless you should talk to your gynecologist and get a pelvic exam to make sure that there isn't anything worse.