I have cervical spinal stenosis and mylomalacia. always after a few hrs of activity i get headaches and have to lay flat. is this common.

Yes: With activity we all get some swelling of tissues. In your case that swelling likely piches on the nerves and spinal cord. I would suggest seeing a pain specialist or neurosurgeon.

Not really: Headaches are not common with myelomalacia but can happen. Lying flat to relieve them sounds more leak a spinal fluid headache. Make sure you dont have something else going on.