On two occasions I've had these, both lasting for around a week and hitting only when I lifted (they lingered for an hour or so, but then went away). This was every session, whenever I strained at all. Both times they went away gradually, and I haven't had them since; I believe it could be a sleep or stress related issue, because foam rolling etc. on my neck only made it worse.
Maybe try avoiding heavy sets and using bodybuilding methods like high TuT and constant tension training for a week - it should go away in time. Obviously go to a doctor if you're still concerned though, it could potentially be something more serious so don't let it go on for weeks. Good luck sorting it out though, headaches can really drag you down if they go on and on.