What Causes Bumps And Irritation On The Outer Labia During Menstruation?

I am 37 years old. I am 5'7" and 207 pounds. I have seen several doctors and no one can diagnose any problems. About 2 or 3 days before my menstrual cycle begins I start to get a bit of an irritation on my outer labia and then the irritation forms a bit of a lump on about day 1 of my cycle and by day 3 or 4 it is about the size of a dime and it itches a lot. Sometimes if I scratch it in my sleep it will ooze a bit of puss or something like it. It goes away by the end of my cycle and then will come back again with the following cycle the same way. This has been happening for approximately 2 years and I would really like to know what it is and if there is anything I can do to get rid of the problem.