What type of rash on a dog's belly its brownish?

My dog has a rash on her tummy that is slightly red but kinda of looks like the area is covered in brown dirt. There are a few pimples also. It isn't actually dirt, she is an indoor dog and we clean it daily. It doesn't itch, and it isn't spreading to me or my cats. It has lasted several weeks now but is not spreading. Right before it showed up we had taken her to the spca and they shaved the area, so I'm betting the buzzer was not clean. Any ideas on what it is and how to treat it?