M.M.
My DS one morning woke up with a bad rash on his back, arms, stomach and I freaked out. I took him to the Dr. and he thought it could be the detergent we were using or his bath soap (I was solely brest feeding). He said he could have become allergic to it even though he hadn't been before. We changed soaps and detergents and that didn't help. Then one day it just occurred to me about a week later that my DH had opened up a package of brand new onesies, didn't wash them and put them directly into my DS dresser drawer.
Oddly enough, that is what it was. He was allergic to the onesies but after we washed them, he was fine. Stranger things have happened =-)
However, my DS also has eczema too.
HTH