What could be causing this swelling in my groin?

About a week before my period every month I get a swelling in my groin, it is about 1-2inches long and very painful. On 3 occasions I have undergone surgery to remove ("excise") the lump, which I was told was an abscess. However it only appears a few days before my period and then slowly disappears a few days after I finish my period. After having it surgically removed it is usually about 6 months before it reappears. One doctor thought it could be endometriosis but when he biopsied the lump he could not find any Evidence of this. I have now started to develop another lump at the other side of my groin, which is behaving in the same way. It is very painful and I worried in case I need another operation. I have PCOS and very irregular periods, not sure if that's a factor. I do not drink, smoke or take drugs and I am not overweight. Anybody got any ideas on what could be causing this? Even my Doctor admits he is baffled.