What could be some reasons that I have migraines everytime I stand up (following a skull fracture)?

See a doctor soon. It is very possible that your severe headaches on standing (after a skull fracture) are due to a brain fluid leak through your fracture. Such leaks can be dangerous and can lead to meningitis if not properly followed and/or dealt with. Typically, early fractures with brain fluid leaks are dealt with conservatively (meds) but if you have had the fractures for a while, you may need more done. See md.

Related Questions

Skull fracture when I was younger, severe headaches/migraines recently. Related?

Possibly, depends. The degree of the skull fracture and associated head injury is an important factor in determining if your current symptoms might be related. The more severe the head injury, the more likely your current headaches might be related. It is known that patients who have severe head injury are at increased risk for delayed sequelae-ie. Hydrocephalus, cognitive issues. You should have this checked out.