I had a headache behind my right eye for most of the morning and afternoon today.
I think it was mostly from tense muscles as my dreams weren't that relaxing.

So pretty sure it was tense muscles.
But maybe also not eating enough food. 
As I noticed that the headache faded more after I got some more food in my system.
So it may be just that time where my body randomly craves more food.

Thankfully the headache faded by the time I got home from work. So woot.