I'll bet money the UTI is back. If you can catch a urine sample from her, your vet can check it to see if the infection is still present. Call ahead to see if you can bring the sample in without the dog - it might save you an office visit charge since he's already seen her for the same problem.
Maybe her uti is back or not completely cured yet. She wouldn't all of the sudden forget her training. But, keep in mind that dogs have accidents, even when trained. She is still a puppy. Is she done with her medication? She might just not be over it yet. Is she having accidents in he same spots or are they random?