years ago i had a staffy that used to get contact allergies around his mouth from his plastic food bowl and from the carpet freshner we used to use, needless to say we stopped using those. He also was allergic to ant bites and we almost lost him a couple of times to this. WE had to have strong antihistamine tablets on hand at all times just in case, but if he was outside by himself and he got bit he would swell up and couldnt breath in a matter of minutes.
So check out simple everyday things like concrete, plastic and even lino in the kitchen, if she lays on her stomach on any of these surfaces stop her from doing so one at a time to try and find the culperate.