I get a sharp but brief (10 secs) headache every time i sit up. i am very rarely ill but suffered a bad back injury last week if that could be related?

Possibly: If your back injury was in the upper back, muscle strain or pinched nerves could radiate to the head and cause a headache. Positional headache is another possibility. Keep a log of these episodes (timing, duration, severity, location, and characteristics) and take it with you to a neurologist. Go sooner if symptoms worsen. http://www.nlm.nih.gov/medlineplus/headache.html