The most likely cause as you mentioned in a hot day and after an intercourse , dehydration can definitely be a cause for hypovolemic syncopal attack in the shower. You may have become unconscious for sometime and once lying down , your conciousness would have recovered. This would be typically dehydration.
If similar episodes re-occur in future after sex and in absence of any dehydration , then it would require an evaluation by your physician in detail including your neuropsychological status.
I hope i have answered your query,
These Q&A’s are for educational purposes and should not be relied upon as a substitute for medical advice you may receive from your physician. If you have a medical emergency, please call 911. These answers do not constitute or initiate a patient/doctor relationship.