sometimes, when i'm doing the last few reps of my set and i start giving it my all, i get these really intense headaches. it's like there is tremendous pressure on the lower left side of the back of my head. it's allready happened to me 3 times in the past week, once while deadlifting, another while leg pressing, and today while i was benching. it usually goes away about a minute after my set, but it definitely does not feel right? any ideas what could be causing it?