Kinda sounds like either a UTI or stones to me, but I'm not a vet.

Antibiotics take time to work, they don't just cure a problem in a day or two especially if it's a bad infection.

I would see if you can get a urine sample and take to your vet. That should help them find any crystals if there are any or detect high PH levels, infection etc.

Out of pure curiousity, what antibiotics did your vet prescribe? Just a personal curiosity you don't have to answer that if you don't want.