I also did TCH, and I know that one of the possible side effects is a rash. My doctor usually advised Benadryl, but that doesn't seem to be helping you.
From the picture, I am surprised it isn't bothering you more, pain or itching or something! You must be a tough cookie. Since they said it it spread or worsened to go to the ER, I would at least call your doctor's office back and let them know whats going on. As far as it being 11 days after your infusion that it showed up, that does seem odd, but you just never know. Those SEs like to pop up and surprise you. Praying that your rash clears up soon!