Unknown Swelling After Sex

After switching to birth control me and my partner have been having sex without a condom. Afterwards I used to experience a burning sensation (we both have no STDs) and would have a small ball (welt?) on the left side of the inside of my vagina. I supposed I had an allergy to my partner’s semen and read that I could build a resistance to it. 6 months later, I no longer feel any burning but the welt on the left of my vagina still appears and is very sore to the touch. It goes away if we do not have sex for a few days and feels like a small lump that I can move around a bit... does anyone know what this is? I’ve never had anything of the sort with my previous boyfriend.