When they have a UTI they cannot help going pee frequently. How long between the two infections? If it was not long, she probably did not get it cleared up completely and as soon as you finished the antibiotics, it reasserted itself. If you have not already, your vet needs to send off a urine sample for a culture and sensitivity so you are sure the antibiotic is effective for the bacteria. If your dog was older, I would say to check for stones, but that would be quite unusual in such a young dog.