I have the same thing. No positives to any allergies. When I was about 21 or so, I got the hives from antibiotics. I am 44 now and I never take them for any reason but about August of last year, broke out the same way as with the antibiotics. Started prednisone. Did not help at all. Dermatology, nothing. Already said allergy came back negative. (and my health insurance seems to not cover any of these tests). Tried a clean diet and it helped but did not cure. Antihistimines do nothing. And I don't want to become immune to them! If I have them one one knee, they show up on the other. I also have had problems with heartburn now.
The sucky part of this is I am perfectly healthy. My bloods are good, cholesterol perfect, blood pressure on the lower end. Even had an ultrasound on my organs and they are perfectly healthy.
Tried new soap, shampoo, and laundry detergent.
Whoever finds out what this is and how to cure it will be my hero.
Good luck to all.