My old Cat has a Bladder Infection again

Coco has a bad bladder infection again. She had one in Aug 2006 with Bladder Stones that went away with S/D. She is almost 16 and is on C/D. She had a terrible cold in August with her Asthma acting up. She was given Clavamox and the Cold went away and a shot for the Asthma. Last time she had the Bladder infection it took 2.5 months to get rid of. She is on her third box of clavamox right now and has one more box to go after this. The Vet did a Ultra sound and her bladder is thick and her Urine yest is Positive.

bumping this up... I cant think of the word I want to ask if the vet has checked for

Thanks,
This is another bad infection. The Vet said she may have to go on lose dose Antibiotics if it dosent get better soon.