If your cat is also vomiting, call your vet immediately.
You may need different medicine or your kitty may have a bigger problem. If your cat is not vomiting and is pooping the normal 1-2 times a day but the stool is soft, consider a probiotic 2-4 hours after the antibiotic. If your cat’s diarrhea is so bad that your kitty is straining, is in the litter box many more times than normal, or if the stool stinks much worse than normal, then your kitty needs to go back to the vet.
Really frequent diarrhea can lead to dehydration; Incredible stink often means intestinal infection and the antibiotics may be making things worse. And, most definitely, if your kitty has blood in its poo or the feces are black, then this is a medical emergency and your kitty needs to be seen.
Antibiotics can do great things, but they also can kill normal, healthy bacteria which are preventing bad bacteria from growing. Any of the above changes and your kitty needs more help.