
Just to add to the list of 'guesses' about what's going on, how about the possibility of a pyometra (closed will mean you don't see a discharge, although you'd see an increase in the size of the tummy) - unless she's spayed. This is something I'm sure your vet will have thought about.
Other possibilities, diabetes, kidney infection or disease - for sure. But don't worry unnecessarily. And keep away from the Internet because a little knowledge can indeed, be a dangerous thing. I'm sure your vet has a handle on what needs to be checked for. And if the antibiotics don't work (it may take time although I like to see some response within a few days - and they might need switching to get a result), he'll have to look further.