Itchy Rash On Ankles, Under Triceps, Breasts And Lower Back. Negative Allergy Test. Any Thoughts?

My daughter plays many sports and for the last three years she has been battling a rash that we can not get control of. The rash breaks out on her ankles, under side of her triceps, under her breasts and lower back. The rash is red and bumpy, flaky, extremely itchy and sometimes oozes. We have taken her to the dermatologist and she has ben tested for over thirty common allergies. The results showed she was allergic to none of them. We have even taken her off of dairy and wheat with no positive results. She seems to break out the most during basketball season but she will also break out some during soccer and track season. Everything we have tried to dry up the rash will always start out great, drying the rash to almost nothing and then all of a sudden she breaks out as bad or worse than before. We make sure her clothing is clean, using a free detergent and she removes all sweaty items as soon as her games or practices are over. I feel as if she is allergic to her own sweat. Any thoughts would be very helpful. Thank You.