Sometimes foods have sugar in them that would surprise you. Make sure you are checking the ingredients really well. For instance, often there is sugar in pasta sauce or juice. My DH is allergic to caramel coloring and we've found caramel coloring in odd things such as canned beef stew.
For the itchy skin, try switching to a filtered shower head. The harsh chemicals from the water softener may be bothering her. You could also give her an oatmeal bath. When I get horrible rashes the best thing for me is Caladryl (the pink stuff) with Cortizone 10-plus on top of it. Also, only use creams that are hypoallergic, scent free, and non-pore clogging on her.
There are also several medical conditions that if left untreated can cause horrible rashes. Hopefully she finds some relief soon! Poor girl.
Have you tried putting chapstick or vasoline, or even some neosporin? I might try that as well.