why do I have a headache?

ive been getting headahes to. but mostly after i have slept hard after taking pain meds. it could be alot of things. caffeine,FOOD .. withdrawel. ive seen this question alot . so im sure alot of us deal with it . mine are not ful blown omg eadaches. but the dull knawing kind that can be annoying. one thing for sure its not my blood pressure ..