We are having a simliar prob with our youngest although he is 2 yrs old! It looks like eczema and last time we switched nappies as a certain brand set it off within hours of trying them. He eczema then became infected - imagine that! Poor chap.

His bottom has flared up again in the last 2 weeks but also on his cheeks, arms and legs..............I am actually starting a food diary........so to cut a long ramble short maybe you could try that and see what makes a diff and what doesn't. So far we have cut out tomoto sauce......

Hope you get to the bottom of it and it clears up soon.
xxxx