Ice pick head aches that are short term (minutes) are often vaso spasm head aches. I get them. They tend to hit me in waves that last a week or two then go away. Head aches from neck strain can be another issue. I agree that you need to get a belly band for the backpack. I used to haul a 3 gallon backpack chasing Canada thistle, Scotch thistle, hounds tongue, toadflax, and a few other invasive weeds. It is amazing how much strain is put on the neck.