What Causes Pain In Lower Left Abdomen And Back Along With Rash On Abdomen?

hello, I had a pain in my left lower stomach going round to my back. the doctor said I had pulled a muscle. I have used a hot water bottle for 7 days and I have a small rash on my tummy and 2 little lumps on my back where I used the bottle.it is very itchy and the skin is very tender, but there are no red burn marks.