A rash is defined as any change in the color, texture, and look of the skin. It can sometimes irritate the skin, causing swelling, redness, and pain, in and around the skin areas where it occurs. A rash can be localized, i.e., restricted to only one body part, or it may be found all over the skin. Rashes that are seen in and around the mouth, occur because of numerous reasons. Here are some of the possible ones.