It could be a reaction. 2 of my kids are allergic to amoxicillin and in both cases symptoms did not show until after taking the meds for over 1 week. They broke out in hives.
I would not have your daughter take another dose before speaking to her doctor. It's best to have benadryl on hand as a precaution. Watch for signs of any signs of breathing difficulty, in which case call 911.
The rash might be completely unrelated, but better to be prepared in case it is an allergy. Good luck.
I just read your question about the possible allergic reaction to Keflex. I had previously taken Keflex for mononucleosis at the age of 18 and and other things during my early 20's and never had a problem. A few years later when i was 28 I took it again. I had a terrible reaction. I had very large raised hives that started on the tops of my thighs, inside my arms and up my neck. My eyes were swollen and breathing became a task. I double dosed benendryl and went to ER. They told me it was a possible reaction to Keflex but since I had taken it in the past most likely I had came in contact with something else that caused it. Well a couple months later I had a bad sinus infection. I took 1 of the Keflex knowing that I may very well indeed have a reaction to it and I sure did! It was terrible. I can't even smell the stinking stuff without itiching now. You might call her physician and see if they could switch her to another antibiotic.