What causes my head to painfully pulse with my heartbeat when stretching or standing up to quickly?

See your doctor: The cause may be benign, but it's possible that you have a mild case of intracranial hypotension: Either you're underproducing CSF or removing it too fast or you have a tiny leak from a little hole or tear in your dura mater. This would cause your brain to sag a little when you stand up. Painful but probably not dangerous. It can be tested for. This isn't a diagnosis, just an educated guess.