I Had A Bad Reaction A Few Weeks Ago From

I had a bad reaction a few weeks ago from a z pak. Severe diarrhea, followed by pure blood. I was told to stop this (after one dose), and put on a high dose of Amoxicillin. I completed 7 days of that and now have a urinary tract infection, for which my Dr prescribed Cephalexin. I am scared of having the same effects as I did from z oak.