Everytime I get into the bathtub my back starts to bother me.?

When I'm submerged in hot water my back starts to feel like I have dozens of ants biting me and it quickly spreads to my tummy like my body is falling asleep. Ive been told that both hot and cold water can do that but it has never happened before, it is only on one spot on my body, and there is no numbness. Is this normal?