Do you have some back problems as scoliosis or so?

I'd be careful with baths. Water can be tricky with backpains, as it gets colds progressively and might make it worse the end. Never really worked for me.

Keep warm though. Which part of back is it? Can you try stretch very carefully and try to slowly, very slowly turn the torso? Could be a "blocked nerve" (not sure of the exact english term, but this is what I get).