Mosito, be aware of the possibility of AMS but don't be hypersensitive to it. It's quite normal for many folks to experience a mild headache at altitude. It's the worsening of the headache or, in particular, other symptoms showing up that puts you in the position of having to assume you have AMS. Those commonly are nausea, vertigo, confusion, trembling, vision difficulty and sleepiness (not fatigue - you'll definitely feel fatigued - but an overwhelming desire to lay down anywhere and go to sleep, and damn the consequences).
So, an average, everyday headache is not cause for reversing course by itself. A killer headache and/or a second symptom are.
Best of luck.
