Sustained a slight concussion/ moderate whiplash 2 weeks ago. Been taking fioricet (acetaminophen butalbital and caffeine) for headache for 8 days, little to no relief when wears off. Typical?

Need diagnosis. The head and neck issues may be interacting, and the neck may be a trigger for the headache, but might consider possibility of disc protrusion, or even thoracic outlet syndrome secondary to shoulder strap. Also, might wonder about intracranial problem. See your doc and try some conservative meds and physical therapy first.
Whiplash headache. If you haven't done so already, i would get into see a good primary care physician for an evaluation. If your pain persists despite a negative workup you may have "cervicogenic headaches" from your cervical facet joints which can be treated through cervical medial branch radiofrequency neurotomies by a qualified pain management specialist. Chronic use of Fioricet (acetaminophen butalbital and caffeine) is not a good idea.
Yeah. . Yeah. Up to a few weeks is normal. The muscles in your neck go up to your head and strain can cause pain. Be patient and stretch slowly when you can. You also might want to consider taking some Ibuprofen if you can as a anti-inflammatory. Good luck.