What Causes A Recurrent Rash On Legs,arms And Neckline?

I have had a rash/hives for about 1 year now. It started on my foot and then started on the calves of my legs and proceeded to my arms, hands, and neckline. I have never had it where clothes cover my skin. Three rounds of prednisolone and one round of prednisone knocks it back but when it is gone, the rash returns. Anti-histimines/cortisone creams do not affect it. It starts as tiny red bumps and as they got bigger they overlap each other. Very little itchiness, more prickly like a heat rash. Not diabetic, thyroid was checked and BP meds changed, non-effective. Biopsy says "granuloma Annulare". What is causing this?