Not a vessel
A blood vessel (either vein or artery) cannot pulsate any more quickly than your heart
So what you are experiencing is more likely a muscle twitching
or spasm. Yes, there are muscles in the back of the head- try raising your eyebrows, and you will likely feel the back of your head tightening too. This is not likely worrisome, but if it persists, call your primary doctor.
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