usually it doesn't affect pain one way or other (recent retear during travel) but last night the hot tub seems to have set me off.)

The worst pain I ever had was after a hot bath, but I think the angle of sitting/lying made things worse. In general, heat helped me every time except one, when I made my hot water bottle too hot and had bad spasms for half the night.
I'm still a fan of the hot water bottle. Just in moderation!

Not sure if you mean your own hot tub or a public one, but most have a TON of added Chlorine, akin to swimming pools but even more concentrated and active due to the heat, to keep human bacteria out. They also can be MUCH hotter than bath water at its hottest, it's like scalding an open sore. Just my guess, but I would suspect the chlorine/chemicals first and the water temp second.