the cause for a condition like this could be due to Anemia
few questions would help to be precise
1)How often you get these episodes.
2)whats your age and gender?
3)When did you first get this symptom?Is it only during shower or while standing for long time any where?
4)Any other symptoms like palpitation
, sweating etc?
hope you find my questions relevant.feel free to ask any more questions