OMG- I think we have the same problem. I am struggling now to try and find solutions to this pain which I have been dealing with for three years. For a long time, I thought that it was excema related to my diet. But for a different reason, I just went off the birth control pill and the symptoms all suddenly completely disappeared (they have never completely disappeared like that, they just calm down or act up). Now, a few weeks later, they are slowly beginning to reappear and I am noticing through more research that they are in fact signs of yeast, even though tests come back negative.
The reason that there is relief during your period and right after is because the hormone levels in your body change throughout the month and affect the flora of the vagina, thereby creating less inviting environments for the yeast. However, you must entirely kill it. I have an appointment next week and I am asking my doctor for a prescription for Diflucan, an anti-biotic to kill yeast.
Also, check out the link below and see if anything rings a bell.
http://www.medhelp.org/forums/WomensHealth/messages/674.html
Good luck!