What Causes Blisters And Redness On Lower Back After Using Heating Pad?

My niece has been using a heating pad for low back pain and noticed a blister, redness in surrounding area but now there are 2 smaller bumps in the surrounding area - non-blistered and feels "smushy"
Obviously she has burned the area and has the heat blister but what would cause the additional non-blistered lumps and "smushy" areas?