Is it possible the day after a chiropractic neck adjustment to have a really bad headache?

Yes. This can happen after any manual manipulation. It doesn't mean anything was necessarily wrong with the adjustment, but it could have caused a muscle spasm that leads to a headache. If you notice any other symptoms, like slurring of words or one sided weakness, see a physician immediately. With a hx of a spinal fusion though, be very cautious with who you see and how much is done. Hope that helps!