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.
There are several causes of headaches. Headaches can be divided into
primaryPrimary amyloidosis
Primary biliary cirrhosis
Primary hyperparathyroidism
Primary insomnia
Primary lymphoma of the brain and secondary.
PrimaryPrimary amyloidosis
Primary biliary cirrhosis
Primary hyperparathyroidism
Primary insomnia
Primary lymphoma of the brain headache
disordersAdjustment disorder
Anorexia nervosa
Asperger syndrome
Autism
Autoimmune disorders
Bipolar disorder
Bipolar disorder
Bleeding disorders
Borderline personality disorder
Bulimia
Chronic motor tic disorder are headaches without a direct cause. These are diagnosed after secondary causes have been excluded. Secondary headache disorders are due to an underlying problem, there are many many causes but some include medication side effects, systemic illness, nervous system infection, tumors, bleeds in the brain or clots in the veins of the brain, and others.
Primary headache disorders are more common than secondary ones. There are several primary headache disorders, over 50 different types. For example migraines, which usually a pulsating throbbing one-sided pain with nausea and discomfort in bright lights that lasts several hours.
That you have some numbness in your arm is a little concerning; while migraines can cause some sensory symptoms, it would be unusual for a migraine to last 8 days. Also, there is a possibility that you sustained some sort of trauma due to hitting your head, that is causing perpetuation of your headache, and obviously the most concerning thing would be that your head trauma lead to some bruising (Contusion) or other types of bleeding in the brain.
Given the symptoms you describe above I think you should be seen by your physician sooner rather than later just to make sure there is nothing serious causing this pain. After he/she examines you and obtain a history he/she may choose to order brain imaging (MRI or CT of the brain). Referral to a neurologist may be indicated for further work-up and/or management of your headaches.
Thank you for this opportunity to answer your questions, I hope you find the information I have provided useful, good luck.