Thank you for using Ask a Doctor Service from Doctor Spring.
I can understand your problem, and I will try to help you the best possible.
The pulsating sensation at the back of your head could be due to pinching/ compression of cervical nerve or occipital nerve (which runs from level of neck to scalp) by overly tight neck muscles,And you have mentioned that you get relief from the pulsating sensation with change in position of head, which points more to the possibility due to nerve compression because as you change the position of the head the compression on the nerve could be released and this relieves the pulsating sensation at the back of your head. This condition can be treated, so do worry, it is not dangerous. You will have to consult with a neurologist/ GP and take help from physiotherapist to teach you certain head and neck exercises that will relieve the compression and your doctor might prescribe you certain anti-inflammatory medication or anti-spasmodic injections that will relieve will help you in releasing the tension around the compressed nerve.
Hope this helps. Feel free to ask if you have any further questions.