What Causes A Non Itching Rash On Arms,waist And Upper Thighs?

HI Doctor, I am a 72 yr old female with very young looking skin, have been told I look in my 50s, however, about 3 months ago i developed a non itching rash around my waist, on the inner part of my upper arms and the bend of my arms and my upper thighs. I am in excellent health other than taking levothyroxine for low thyroid. I have tried cortisone cream that helped a little, right at this time I am using Cetaphil to bathe with and use the moisturizer after. I have wondered about using a powder without talc instead of moisturizer. Have you any suggestions what this might be? I also take a multiple vitamin and 2000 mg of C each day.