Rash advice

My 9 month old has had a rash on her diaper area for about 5 weeks now. It's not a diaper rash, as I've exhausted multiple medications with her doctors over the last month. Nothing works. She's been on nystatin, nystatin w/ a steroid, clomatrizole, regular diaper rash cream, extra strength cream, and Vaseline. The doctors say that it's not a yeast infection or UTI & are now possibly thinking allergy. However, they said that no doctor will do an allergy test on such a young baby. I've already withheld solids with no change. My next steps are to switch to soy formula in case she developed a milk allergy, and also to switch her diapers in case she developed a sensitivity to those. I also haven't changed her diapers nor detergent or anything like that so I know it's not a reaction to something new. I'm currently waiting on a dermatologist appointment also as her doctors have said there's nothing else they can do because they don't know what's causing the rash. Anyone have any advice or had a similar thing happen to their babies?