It's the hot water! I get like that in the shower because I love the water boiling hot. I come out and my temperature must be like 40 degrees. I think it's something to do with the heat messing with your blood pressure. Keep it cool, and try to stand up slowly and grip onto the side of the bath incase you fall. Does it happen any other time when you're doing other things?
Does it happen any other time when you're doing other things?