Your right. A neck problem can cause headaches. Have you had your neck checked out? I would definitely do such. If you leave it unchecked and untreated, the problem could worsen. Hope you feel better soon and see your PCP!!
Yes that could have something to do with it
Yes it could cause a headache. Your neck may be out of alignment from the injury and you could have a pinched nerve.
What kind of injury did you have, did they take ex rays? You have to make sure you did'nt break any of the bones in your neck or have any bulging discs. These two things can also be causing your pain.
If you have'nt had any tests, do so, and if they come back fine you might want to consider seeing a Chiropractor.