Constant headache in the back of my head?

This sounds like a "tension headache". which affects the back of the head. mostly causing all of the symptoms you are describing, taking tylenol will help becaue it's the best aspirin for tension headaches. but make sure you get enough sleep and are eating on a regular basis and also staying hydrated. if those don't work go and see your doctor. he or she might prescribe you a special medicine just for tension headaches. i hope this helped!