What Is The Treatment For Swollen Painful Left Labia?

Hi since sunday morning i have had a swollen left vaginal lip. It is swollen right into the vaginal opening. It is sore to walk and sit. I have noticed this since having intercourse with my boyfriend at the weekend. I do not have an STI as i got tested for these before and i 100% trust my BF wouldn't cheat. What do you think is wrong? Im 22 years old, 5:10 and weigh 12 stone