I am not a neurologist and this is not my speicalty.
It sounds like you are describing a "thunderclap headache" which people often say is the worst pain they have ever felt. I'm glad your MRI and spinal tap were negative. They were looking for the most severe things, too. Most of these headaches turn out to be "nothing serious" and are treated much like migraine.
You need to see a neurologist that specializes in headaches. A slim concern is a small tear in the dura surounding the brain/spinal cord and something that may be called an "aseptic arachnoiditis". This is an inflammation (not infection) of the space around the brain which can be associated with a tear. This type of headache may be associated with bowel dysfunction like explosive diarrhea.
I am assuming here that the diarrhea is unusual for you, too, is this correct? Have you been having the abdominal pain and diarrhea only in connection to these headaches?
Here is a link:
Does the description on this site sound like what your are experiencing?
Did anything unusual happen to you just before the headaches began?