What Causes Pea-sized Lump On Outer Labia?

I have a pea-sized dark purple/red lump on my outer labia that appeared about 3 years ago. It is quite prominent and changes in volume sometimes, occasionally becomming tight with fluid and sore. It is not generally painful but is unsightly and a concern to me as I dont know what it is. I have my gyno look at it a couple of yrs ago but she didnt know what it was, and said it was probably nothing to worry about. Any ideas?