Oh Tulip thats awful!
Showers really tire me out too, especially if I have to wash my long hair.
I have POTS/OI so I think its a combination of- any prolonged static standing (not moving around), blood pooling in your feet/legs, warm/hot temperature, raising arms above your head....
I shower in the evenings most days as that is when my POTS is least severe.
If you used a shower chair and put your feet up would that help? Do you get blood pooling in your legs? (do they go red/purple and puffy?).
I forget, do you have POTS/OI? Perhaps before a shower you should load up on fluids and salt (like drink 500ml- 1 litre, make DIY rehydration mix or use a sports drink or vitalyte) to boost your BP.
Do you get pre-syncope? (pre-fainting) symptoms? I usually get heaps and heaps of nasty symptoms before I actually would faint, so I've not really properly fainted in a long time. I get really dizzy, nauseous, hot/cold & sweaty, woosy/light headed, my hearing goes faint & vision starts to grey out. When I get these symptoms I have to lie down or sit down with my head between my knees in a hurry. It sounds like you mustn't get much warnings signs though, that sucks.
You might have NMH- neurally mediated hypotension or postural hypotension, which is causing a big drop in BP when you are upright/standing and so your body responds by making you faint so you become horizontal in a hurry.
