Is it possible I have a brain tumor? I've had this horrendous pain originating from a spot on my head had a CT scan done no contrast-clean

Tumor Unlikely. While a very small tumor could be missed on ct scan, a very small tumor would not likely cause a severe headache. Consider migraine.
Consider MRI. A non-contrast ct scan is limited in what can be detected (large lump, recent bleeding, etc), but an MRI of the brain (order it "with and without contrast") is much more detailed and gives a cleaner picture. If you have rapidly escalating pain, you should seek immediate medical attention as these can be signs of serious bleeding.