Health is draining even after leaving the Soul Cairn
When you do the quest to enter the Soul Cairn, you have to 'soul trap' your soul so it doesn't take that damage if you side with the dawnguard. It is possible that when you turned into a human, you needed to do this in order to enter, and that turning back into a vampire didn't change you back to being able to do it without doing the human side of the quest.