I don't have any suggestions about herbal remedies, but I used to suffer AWFUL menstrual cramps and I found that Aleve worked even better than Motrin (and before Motrin there was nothing that worked. My mother's doctor used to tell her to drink. Seriously.)
If I remember, I used to take one at the beginning and then one 8-12 hours later. It was slightly higher than the recommended dose but it usually got me through that first 24 hour hump of pain. Oddly, it was after starting the Aleve that my cramps started subsiding, like it had some lingering effect, and now I mostly get by with nothing, or something minor.
Good luck. I remember hearing somewhere that having bad menstrual cramps can be like giving birth. It has been a while since I've done either, but I can believe it. And nobody is giving you an epidural for your "time of the month!"