Blister/Cyst on Labia Majora?

So I've had this blister like cyst on the outside of the labia majora and I can't figure out what it is. I'm only 18 and still working up the nerve to just go to the on campus gynecologist about it but I'm just trying to get an idea of what it could be. It's not painful except when it pops and blood and pus come out for a while. Then it scabs up , goes away for a few days and comes back. Theres only one but it gets kind of big and it feels like theres a pebble under the skin. Can anyone help me out with an explanation /: