You may have been experiencing hypoglycaemia. It may have also been the shower; if it was too hot it may have precipitated the fainting by temperature effects on BP or you may have been dehydrated.
I would like to know:
1. Do you urinate frequently?
2. Are you hypertensive (high BP)?
3. How old are you?
4. What is your diet like?
5. What exercise/activities do you do?
6. When was the first time this has happened?
If you are concerned, you should visit your GP. If diabetes is a differential they can do some blood tests to see if it is indicated.