Could be exercise-related, but one headache is a pretty small sample size. It might help to keep track of when you get them and what you've been doing/eating/drinking, etc. That might help to isolate what factor is a probable cause (you mentioned medication as a possibility, for instance; could even be changes in the weather or any number of things).
Hope you figure it out and get rid of them soon...and congrats on pushing it a little harder!
And now that you don't have to be perfect, you can be good.