I would have to agree with everyone about the spaying.

But...... have you even given thought to maybe she has a kidney stone and is maybe trying to pass it? Kidney stones are extremly painful, but once passed you are fine. Does she drink a lot?

Your best bet all around would be a vet check up. Tell them about the screaming, and they will run tests for everything, but will find a soultion!

Good luck, and let us all know what the problem actually was!