Is there an explanation for extremely frequent and extremely painful headaches (i drink water regularly so not dehydration)?

Headaches and water: Check your sodium by having a physician order a blood test for it. If you drink a lot of water, and not ingesting a lot of sodium, you may have a low sodium level. Why do you think you are preventing dehydration by drinking large amounts of water? Are you having diarrhea, being exposed to a hot environment or urinating a lot? If not, you probably such drink to thirst.