Hmmm. When I developed similar symptoms in a similar situation, my doctor said it was most likely a new allergy to the penicillin. (Of course, I haven’t *seen * your rash, and am *not * a doctor, so this comment is worth precisely nothing.)
Long about 1985 or '86, I was taking amoxicillin for bronchitis, I believe. I was near the end of the prescription , and had gone back to work, when the itching began. My hands turned beet red and developed donut shaped hives that stood 1/4 inch high, and visibly stood out, being pale white against the red ov my hands. My fingers swelled up like sausages; I was unable to even bend them. This happened in less than 3 hours. I had to drive home, using my wrists against the steering wheel , since I couldn’t bend my fingers. I went to my parent’s house , and my mother , being the wonderful woman she was, said she simply couldn’t drive at night to help me get to the hospital. By this time, I was getting hives on my scalp and back, and breathing was becoming difficult. I called my father home from work , and he DID come home and get me to the ER, where I was given several injections and a perscription . I was told if I had waited another hour or so , my throat would have closed completely, as there were hives IN my throat , and I would have suffocated.
I have taken amox and penicillian many many times in my 25 years - with no ill effects. A few years ago, I was taking one or the other for an ear infection. With one day to go, I developed an awful rash - right under my left boob. I called the doctor who told me to take Benedryl. It helped the itch but because of the unfortunate location of the rash, it didn’t go away for over a month. Other than being rather annoying, the reaction didn’t get any worse. I am now not allowed to take either. Luckily - I haven’t had to since then.