Skin Problem or What?

I have been dog sitting our neighbors dog for about a week, while they are on vacation. I took the dog on a walk last week and it started to rain so I took shelter under this rusty awning until it passed and then we went home. I dried the dog off and put him in his crate for the night. The next morning I went to let him out and noticed a red irritated area the stretches from the bottom of his neck to the start of his tale. This was about four days ago and it has started to look better. But I still want to know what it is. Any Guesses? I know it is hard to tell without a pic but my camera is broke.