I don't know what the cause is?

Just last night I noticed a rash on my sons back....just his back and not really around his butt. So I know he has been on prunes.....today was his seventh feeding on prunes. I also know his matress in his crib is plastic and sometimes he sweats like pig and I will have to smell the sheets to see if it was sweat or if he just pee'd the bed because both "wet" spots look somewhat similar. I don't know how to figure out what it might be. Like i said.....it is just on his back? You would think a food allergy would be more instant then take 7 days to manifest. And if it is his plastic matress...then what do i do? Get a Sham-Wow to stick undert the sheet where he sleeps? haha