In the back left of my head i feel a vein pulsing quickly, much faster than my heartbeat for 10-15 sec. 2-3x a day for the past 3 days. what is it?

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.

Hard to say: Could be a vein under the scalp or could be muscle spasm.