I think I agree with the tension diagnosis. But to add my uneducated twopence-worth, how well hydrated are you _before_ running? Obviously not a good idea to gulp down bucketloads immediately before going out, but do you drink sufficiently on a day to day basis?
I speak not as an expert but as a reformed drunkard - once a dehydration (hangover) headache's set in it's very hard to shift, irrespective of fluid intake thereafter.