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 primary and secondary. Primary headache disorders 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. With a normal CT scan, many of the secondary causes can be excluded, though use of contrast, or an MRI, is more definitive.
Primary headache disorders are much more common than secondary ones. Migraines are the most common. It sounds like your headaches are very transient. There are a few primary headache disorders that lead to transient pains. In primary stabbing headache, sharp or jabbing pain in the head occur, either as a single stab or a series of brief repeated volleys of pain. Primary stabbing headache often occurs in people with migraine. The pain itself generally lasts a fraction of a second but can last for up to one minute in some people. Another type of stabbing headache is called paroxysmal hemicrania. This is marked by episodes of stabbing or sharp pains that occur on one side of the head and may be associated with eye tearing or runny nose. Episodes may occur several times and last 30 seconds to a minute. If your pain starts at the back of your head and shoots forward, it could be occipital neuralgia as well. Occipital neuralgia is caused by irritation or injury to two nerves that run from the upper neck to the back of the head. The irritation could be due to neck trauma, pinching of the nerves (by muscles or arthritis), and other causes. Symptoms include a piercing sharp pain that travels from the upper neck to the back of the head and behind the ears. It is usually a one sided pain but can be on both sides of the head.
Evaluation by a neurologist may benefit you and is recommended in order to better understand the cause of your symptoms and what treatment would be effective.
Thank you for this opportunity to answer your questions, I hope you find the information I have provided useful, good luck.