When you say rash do you mean a skin surface reaction like small blisters to the hotwater bottle rubber,
or is this an underskin surface problem called "Toasted Skin syndrome" which is what you get when you spend too long next to a gas fire for long periods of time. What my Scottish great Aunt called Tatty-Tartan? the medical term is "Erythema Ab Igne" if you look on google pictures...
Like this image:
what i called giraffe skin, when i was younger and 1st came to the UK. I was so cold I would sit next to the fire most evenings to keep warm and the sides of my legs both looked like this.
It does take absolutely ages to disappear, and i have read that in some people it never disappears because you have cooked the blood vessels too much.
The only way to stop it is to remove the source of heat.
Mine took a couple of years to vanish, but i was only a fit and healthy but cold teenager in those days.
If your reaction was a dematitis allergic reaction to the hotwater bottle rubber, then knit or crochet or buy a woolly cover for the hot water bottle.
Dilute the hotwater bottle with cold water to bring the temperature down to about 40 degrees,
You could measure this with a thermometer and a measuring jug to get the right balance of boiling water and cold tap water.
Your body temperature should be around 37 degrees C or 100 degrees fahrenheit average.
Boiling water is 100 degrees C or 212 degrees fahrenheit. more than Twice as hot !!
It does cook your tiny blood vessels causing them to expand and burst.
There is no cure for the Giraffe skin but to leave it in peace to recover by itself hopefully.
Perhaps speak to your GP about getting referred to a pain clinic, you certainly cannot go on using the hotwater bottle as a pain reliever at high temperature.
These giraffe rashes can lead to skin cancers, so please be careful Badger.