See a chiropractor NOW!!!
Have you seen a chiropractor yet??? For me it turned out to be caused by a strained muscle in the back of my neck (over simplification). I was in some SERIOUS pain every time I lifted heavy... legs - bench - - didn't matter what exercise. Just like what you said, about the 4th or 5th rep it started pounding. This persisted and got worse for 2 weeks before I sought out my chiropractor. Also like you it first happenned when I lifted in the afternoon (I'm usually a morning lifter). Coincidence? Who knows. I had a forced 2 week sebatical from training. After seeing him daily for the first week, then 3 or 4 times the following week, I finally got rid of them.