I have to slowly help patients and the elderly get up because this is common , when they lay down and get up, they get dizzy and are at risk for falls. Because you're young its' transient and I doubt it's from a condition since you mentioned getting out of a hot shower, dehydration also contributes to this. Make sure theres handles to hold onto to so you don't fall. It's called orthostatic hypotension if you want to read about it.