For two days now, I have a mild-moderate headache only when I violently shake my head up and down. Just nodding or shaking head left to right im fine.

Stop violent shaking. Most people will get mild headache if they violently shake their head up an down. Stop it. You might have more tension in your neck than the average 18 year old, or maybe some chronic sinus stuff. That could 'tighten you up' so that you could get a headache from violent shaking. If you're otherwise feeling well, don't worry about it.