D.P.
I have heard of carpet cleaners causing break outs and rashes in some children. If it continues consider it a possibility along with a long list of things you go through one at a time. Lotion, soap, detergent, air filters, bedding, fabrics, a new food...
And for anybody out there using benadryl, I know it does work but...
When my son was 5 we gave him benadryl for allergies and suddenly he started breaking out in hives. It took a good week of dosing him up with benadryl to relieve the hives before I asked the doctor if it could be the benadryl causing the outbreaks. Sure enough it was. The artifical coloring was most likely the cause(red # 5 or whatever they call it-it's in so many things).
The best thing you can do is log EVERYTHING you do.
Good luck!