I always feel weak after taking a bath or shower?

This isn't something that just happened recently, this has been happening for probably a few years now. Almost everytime I get out of a shower or bath, my heart starts to pound, my head starts to feel very weird, and I start to see everything go black, as if i'm about to faint. I usually lie down on the bathroom floor for about 5 minutes until it passes over. There was one occasion where I actually did faint, the second I stepped out of the shower. Usually, if I drink a glass of juice or soft drink before I get in, I feel fine afterward. This usually happens when I take naps during the day time too. If I nap during the day time, I wake up feeling sick and very dehydrated. What could be my problem?