I have a hard lump on my bikini line, under the skin

I have a hard lump on my bikini line, under the skin .. it hurts when I have underwear rub against it. It only come when I'm due for and have period. It's really annoying. Not sure if I should be concerned as it goes away. It is hard, kinda sore to touch, uncomfortable. It doesn't have a head and doesn't have anything come out of it. As it goes away each month I don't know if it's of concern