What Causes Itchy Rashes Inside Of Thigh?

Since the age of 17 I have been getting this rash that itches like crazy on the inside of my left thigh, but I only get it when I get my period then it goes away like a week or two after my period has ended. I am now 26 and it is still happening every month. Even when I was pregnant with my son I still got it every month when I was supposed to start. It has been driving me crazy for 9 years now and im ready for it to be gone and other doctors have no idea what it could be. PLEASE HELP!