I know exactly what you are talking about....I get same pain in same area. Your issue could be different from mine, but I'd get the exact same sharp pulsating pain (yep in rhythm with my heartbeat) along the back left portion of my neck. I knew it wasn't from lifting because I'd been only boxing....I stop boxing for awhile and it was still throbbing here and there...mostly at night.
believe it or not mine is associated with my sinus allergy issues. I didn't figure it out until my wife started complaining about the same pain in same back of neck area (and she has no sinus issues at all, she just got a lil stuffy while we were on vacation and pick up an sinus infection..etc) basically she went in and got med and was all good..no more neck pain..none sense.
It made me think... since I have allergy issues, so when mine kicked in I noticed my sinus issues were kicking in some. I use sinucleanse to flush sinus area twice a day and no more neck pain.
Again your case could be different, but the area and details of what you are describing is exactly the pain I get when my allergies kick in..etc.