Seems like headaches are a normal side effect to taking fludrocortisone. And it might raise intracranial pressure, as Alisha12 mentioned
http://pubmedcentralcanada.ca/pmcc/articles/PMC3390096/
"Fludrocortisone increases plasma volume in patients with POTS
[31],
[34]due to salt and water retention and also sensitizes the blood vessels to constriction.
[35]Some physicians combine salt tablets with fludrocortisone in order to ensure its effectiveness, although this must depend on salt intake.
[36]Patients on fludrocortisone should be given magnesium and potassium supplements due to their concurrent depletion. Also, fludrocortisone has the potential to raise intracranial pressure, therefore it cannot be used in some cases, which involve the brain. Like beta-blockers, fludrocortisone reduces renin levels and may be counterproductive for POTS patients with low renin levels.
[30]Some patients suffer severe side effects, particularly severe headache.
[36]"