I think, as diabetics, we have to be careful how we attribute symptoms such as a headache.
My first thoughts are always towards to diabetes being the cause of any ache and pain, even when I've tested and found my numbers to be good. I do try to sieve out the general symptoms that could be found by those who don't have our problem unless they are persistent, such as your headache recurring daily.
I'd guess that you answered your own question with the 'had a drink of water' bit and was probably bit of dehydration.