Head ache that comes and goes.?

My head have been hurting for probably three weeks now, it comes and goes. My hair has mostly been in a pony tail, maybe its that. Im not sure, i dont want to take medicine for it because it's a waste. Sometimes my head hurts only on the left side of my head. I think its also my vision, since I don't wear glasses when I need to. What can it be? Im getting a little bit worried.