I Am Allergic To Sulfa, I Have Been Taking Vitamin

I am allergic to sulfa, I have been taking vitamin cosamin ds joint health supplement and just noticed it has chondroitin sulfate. I have had a rash on my legs stomach and arms for quite some time. I went to the dermatologist and she did not know what it could be and just gave me cordran 4 mcg/cm2 tape to apply on rash it has not really helped at all and still have rash, the more a scratch the more rash spreads. I stopped taking the vitamins for about a week and I don't see a change yet. Could these vitamins be the problem to my rash. Is sulfa the same as sulfate???