If I lay down longer than 8-9 hours (asleep or awake) I develop a severe headache. Worst when lying down or bending over at the waist. Whole head hurts, but feels worse at back bottom of skull. Advice?

Positional headache. Postural headache is usually associated with low intracranial pressure. Headaches with a postural component need to be evaluated to exclude a lesion in the posterior fossa or a condition with low CSF pressure. Rec.: see headache specialist for further investigation.
Could be nothing... But i recommend you get examined sooner than later by a physician. This could be just positional, but it could also represent increased pressure or a mass within the cranium. I would recommend you just get this checked out to be safe. Once you get a clean bill, work on stretching and core exercises if nothing else comes back positive. Keep an eye on your blood pressure.
See a doctor. It could be a variety of factors, but a neurologist or headaches specialist needs to be seen who can evaluate your health history, meds, and run other tests. I would not wait too long.