Tylenol, not asprin.
Okay, allow me to be overly technical for a little bit; after all, I did get my masters from a department that focuses on cardiovascular physiology.
Anyway, the reason that you get headaches from powder (probably the fumes, you would have to stick powder in your mouth to absorbe enough nitroglycerin to give the above effect;
Under no circumstances should this ever be done!) is because nitroglycerine degrades rapidly into nitrous oxide (NO) in the blood stream. NO is a very powerfull vasodilator that effects the smooth muscle of the vasculature, causing them to relax, and therefor, expand. This is a great thing if you are having a heart attack, but it gives you a serious headach because it also dilates the vessels of the brain.
Asprin is a member of the class of drugs known as NSAIDS (non-steroidal anti-inflamatory drugs). These drugs lower swelling, and also thin the blood, slowing the clotting reaction. If you get a headach from reloading, and then take asprin, you will probably make the headach worse, not better.
My $0.02 would be the increase in ventilation that others have mentioned, and some tylenol for the pain, if necessary.
D
p.s. I can provide overly boring and hard to understand chemical reactions for how all of this happens if you would like, but that sounds way to much like work for right now.