I am getting at least one painful boil in vaginal lip every month. max in outer lip or clitoris. this has been happening for more than a year. why?

Bartholin gland: The recurrent "boils" are present in the Bartholin gland which is inside the labia. It is a small collapsed sac unless infected. Antibiotics can treat the infection, but the most effective solution for recurrent infections is evaluation by a GYN MD who may suggest opening the gland up after drainage and surgically marsupializing the gland itself to allow drainage to prevent further infections.