This is pretty normal, these are referred to as exertional headaches. You mentioned that you got these during your "hardest" sets, which is when most people experience them because they are pushing their body to the limit. Personally, I use to get them on my heaviest deadlifts.

Basically it is caused by a build up in blood pressure, which can usually be avoided by better breathing techniques. It is common for the pain to be focused on the back of the head and upper neck which seems to be what you are describing. You can find more on them here: http://www.headaches.org/consumer/to...xertional.html