I am having terrible headaches after a anterior cervical discectomy plus spinal fusion. related?

It may be related: Especially if the headache is positional -worse when up and about and better when lying down suggesting there may be a leak of spinal fluid from a dural tear. The dura is the outer layer of the spinal cord and this tear is a risk of surgery. See your surgeon. Other reasons could include muscular v medication v another disc issue.

Collar: If you are wearing a c collar after surgery, take it off. These often keep the neck in a very uncomfortable position that may cause spasms and headache. Narcotics may also result in muscle tension headaches: Percocet is bad about this. Wean pain meds or try different one (no nsaids as they block bony healing). Some gentle stretching may also help. Discuss with your surgeon if not better.