That's how it all started for me too. I have been diagnosed with Graves Disease and took RAI. After a couple of months my eyes started to cross every morning for about 5-10 min each day and then return back to normal. Then eventually my right eye is permanently stayed crossed inwards towards my nose and now I am waiting for my strabismus surgery.
Mike,
I would say since it's intermittent, then there is not too much worry. The headache and the double vision could be part of a migraine "picture." You should have a regular checkup just to be on the safe side.
Best wishes,
Timothy D. McGarity, Medical Doctor, Ophthalmologist