That happens to me every now and then also. I don't know what it is from. Just try not to close your eyes too much. That is all I can say!
Not much help, I know.

Could be the water is to hot or you have low blood pressure or some clogging of the arteries to your brain might be worth having a check up.

I think i've heard that anemia can cause this too.

What about vertigo?

Maybe you didnt eat enough?

Maybe you haven't drank enough fluids?

This happens to me a lot. The combination of the humid temperatures and biological factors can have that effect. Often if I don't eat or drink as much as I should and I have to stand for long periods of time that happens to me. You said you're normally in the shower when it happens and i'm assuming you take a shower first thing in the morning before breakfast. All it is is a low iron and blood sugar count. If it continues you may be annemic and you should get it checked out.

Are you pregnant? Im 13 weeks and this happens to me frequently. Someone suggested having peanut butter to stop the dizzies, I guess its high in protein.