I Have Reoccurring Body Rash, Which Is Very Active Right

I have reoccurring body rash, which is very active right now, along with night sweats and cancer sores in my mouth. The rash has been occurring over 8 months, goes away when on prednisone, but keeps coming back. I have been told it must be an allergy and to take over-the-counter antihistamines but I don't have any normal signs of allergy (runny nose, wheezing, itchy eyes). Any ideas?