There is something like a blister growing on the labia majora, red and swollen

A blister-like thing growing on the labia majora accompanied by redness and swelling is usually considered to be caused by herpes. It generally requires treatment with antiviral drugs such as acyclovir, or topical application of erythromycin ointment. Pay attention to keeping the perineum clean and change your underwear frequently. Avoid sleeping together during treatment and do not scratch the affected area with your hands.