Can fludrocortisone dilute blood too much and cause shortness of breath without causing pulmonary oedema or heart failure?

Unlikely: Fludrocortisone is more likely to cause electrolyte changes or changes in glucose metabolism. Vascular volume changes significant enough to cause dilutional anemia (to the extent that oxygen delivery is affected) or congestive heart failure (if not already significantly predisposed) would not be likely. Notify the prescribing physician so appropriate follow up is arranged. Good luck!