What Causes Pain In Vaginal Lip During Menstrual Cycle?

I have a sharp knife pain around my vaginal lip. It is on the same spot each time I get it, which is usually a week or so before my cycle is due. I am 43 years old and I have seen my PCP and Gynecologist about this and nothing is showing at all. The pain is sharp enough to take my breath away and comes with the movement of my muscle in the vaginal lip area. For example the motion when I lean over to wipe off when using the bathroom, or when I stand up from a sitting positon. It usally last a few days. There is no pain with urinating and wiping off in the area. Please help, the pain has actually put me into tears at times. I have been told to take luke warm baths and rub mineral oil on the area. This has not helped.