Persistent painful headache for 3 days. hasn't gone away at all. sore achy stiff legs for same 3 days. what could be causing this?

Head ache: Head aches can have several causes some serious others not so much. Achy legs and headache might be associated with a systemic disorder possibly virus or other . It would be difficult to give you a straight answer for such complaints and could possibly be serious. Best to see your doctor

Many things: It is hard to say whether or not the headache and achy stiff legs are caused by the same thing. You need a full history & physical, and possibly lab to make a diagnosis. Headaches usually fall into 3 categories: 1. muscle/tension 2. migraine and 3. other. The third category includes many uncommon and rare conditions, some serious and some not. As usual, a physician consult is indicated.