Mine stopped after a few months on raw. But it is perfectly fine becuase I am still ovulating. As long as you are still ovulating, there is no problem. Trust your body. If you are no longer ovulating, then it would be cause for concern and I would suggest consulting someone with experience with raw diets and womans issues.
I've always had very short and light menses with no cramps or problems at all so this was not a surprise to me.