@GracieGirl,
There are cramps and cramps. Prostaglandins are said to be involved as well as a narrowed cervical canal. Mine, which were often severe to the extreme (I've vomited in all the best places), didn't go away until a gynecologist prescribed birth control pills to ameliorate them in my twenties.
For all I know Pemerson may be right that yoga and being athletic in general has some bennies for it, but that may be anecdotal - I haven't researched whether muscle tone has anything to do with it as a cause. Perhaps that is additive to what else may be occurring physiologically. If your cramps are really getting to you, GG, see a gynecologist at least for a talk. Going to a gynecologist may involve an exam you don't want to have yet, but I'm not sure about present day practice - but they are the experts even if it is only for a talk.
This site, which has several pages to look at with an ad to get rid of for each page, is useful re what is occurring.
http://www.medicinenet.com/menstrual_cramps/article.htm
* adds, my twenties were a long time ago - birth control pills may not be what is prescribed now.