Weird cramps

Last Tuesday I started experiencing really painful pre period cramps. I wasn't due to start my period until Saturday 6th December but since my cycles can vary a few days either side of the 'due date' I assumed my period would be starting early again this month. However my period didn't arrive as expected and the cramps feel like they're getting worse everyday. At one point today I was nearly doubled over in pain. Can anyone shed any light on this? Really hoping my period will arrive soon.