What Causes A Red Rash On The Chest After Sun Tanning?

I have a red rash in the middle of my chest after sun tanning. my whole body is tanned and looks fine, but the middle are of the chest, in between the 2 pectoral muscles is very red, and it is painful, the skin is starting to shed now, i am worried, what could this be? along with that, there is discoloration of my skin around the chest area, it almost looks as if the sun didnt tan that are and now it looks really pale and white compared to the rest of my body.