Excruciating lower back pain during period?

Every month(along with my monthly menstrual cycle) I get excruciating lower back pain. I have heard that women get back pain during their period so I thought it was normal but it just keeps getting worse every month. It lasts for the first 2-3 days of my period. I can hardley walk, it hurts with each step I take, I can't bend over and when I turn or move suddenly the pain is so intense I feel as if I could p out. But then it just goes away...like magic and does not return until the following month when aunt flo visits again. Anybody else have this happen? Anyone know why I get this pain? I don't have health insurance and I really can't afford to see a doctor as I'm sure they will want x-rays and such...I know there isn't much else to do besides see a doctor but it only hurts once a month! I don't understand...