I really think that MRI and MRA are required emergently. We have to rule out any vascular anomaly like aneurysm, AVM etc in the head if we have ruled out meningitis
from CSF examination.Neck stiffness
seems to be secondary to CNS problem.
If Both are MRI, MRA is normal We have to rule out third occipital headache
which is due to neck facet joint problem and is easily treated by few interventional pain procedure. I will like to keep migrain also in differential. You need to visit a pain physician or a neurologist
for detailed history and examination.