answer:What’s the hot water bottle’s exterior covering? The part that touches your skin? If it’s non-porous (like rubber), then you’ve likely chapped your skin by preventing your sweat from evaporating. Alternately, you could be sensitive to latex/rubber. I won’t treat you to the alarmist website I took this from, but: “If you have mild skin reactions from latex, anti-inflammatory medications may help relieve symptoms.” Personally, I’d just leave it the heck alone for a few days, and hope the rash goes away. You should be able to buy an electric heating pad or blanket very cheap if you don’t mind disinfecting second-hand!