Suggest Treatment For Itchy Knee And Thighs

I have an itchy rash on the backs of my knees and thighs. Only getting mild relief from antihistamine and hydrocortosone cream. Seems worse at night. Have had it for almost a week. Had this once before and it spread over most of my body. Was given prednisone, and it went away. Have had past dx of autoimmune issues including hashimoto's thyroiditis. I'm 45, about 50# overweight, had necrotic gall bladder removed 6 months ago. Mild achy joints intermittently. Looking for relief from itching. Not currently taking any medications. Take multi vit. supplement.