Suggest Treatment For Swelling In Right Side Of Groin Area

I've got a swelling on the right side of my groin area. It comes about 8-9 days befre my period and lasts for about 9days. it's happened for about 1.5yrs and I've seen the GP a few times about this. One doctor though it was a hernia and I got a scan for that but there was no hernia. it's quite tricky cos it only appears at certain periods in the month. it is about 2cm wide, hard and painful. Can you help?