Rash on back

I noticed this rash on my baby this evening. Its only on her back. I have no idea what couldve caused it. I havent changed soap or detergent, which i would assume a rash from that would show up all over her anyways. And as far as foods go the only new thing she had was oatmeal. But again i would assume that would be all over her body. Idk though. Any ideas? Should i call the dr