Thanks for using the forum. I am happy to address your questions, and my answer will be based on the information you provided here. Please make sure you recognize that this forum is for educational purposes only, and it does not substitute for a formal office visit with your doctor.
Without the ability to examine you and obtain a history, I can not tell you what the exact cause of your symptoms is. However I will try to provide you with some useful information.
After head trauma, a headache could be just due to superficial trauma, like a bruising. However, the concern would be that there has been a small bleed within the skull. The bleed could be outside the brain and its lining, what is termed and epidural hematoma, underneath the lining of the brain (a subdural hematoma) or within the brain itself, what is termed a contusion. With particularly subdural hematoma, because it is a bleed that occurs from vein tearing, the headache and other symptoms may follow several days after the trauma rather than occurring immediately. After head trauma, a headache can occur even without bleeding in the brain, in that case it is benign and will usually resolve after a few days. When neck pain is an associated symptom, concern would be for a tear in one of the neck arteries (called a dissection).
Having a scan of the brain is necessary. A CT scan of the brain is usually sufficient to detect bleeding in the brain.
Thank you for this opportunity to answer your questions, I hope you find the information I have provided useful, good luck.
Yes, go to your ER where you can get the help you need quickly.