Maybe start cranberry capsules and a little vitamin C plus make sure she drinks well. Is she spayed? If not, any signs of pyometra or may she possibly be coming into season?
I found a blurb on web that indicated that septra is NOT for initial UTI.
"Urinary Tract Infections: For the treatment of urinary tract infections due to susceptible strains of the following organisms: Escherichia coli, Kelbsiella species, Enterobacter species, Morganella morganii, Proteus mirabilis, and Proteus vulgaris. It is recommended that initial episodes of uncomplicated urinary tract infections be treated with a single effective antibacterial agent rather than the combination." http://www.druglib.com/druginfo/septra/
Cephalexin is contra indicated if there is any sign of kidney failure, not out of the realm of possibility in a senior dog. It could make a dog with compromised kidneys sicker.
Therefore, I wouldn't give anything stronger than the cranberry capsules and vitamin C, which alone may control this if still at the subclinical stage. Your vet can help you tomorrow. If she is prone to UTIs, the cranberry capsules may be more effective at preventing, rather than treating, UTIs, but it is worth a shot.
Or I might try fish amoxi, especially if okayed by your vet over the phone.
I, too, would be concerned. The lack of appetite in particular. What is her temperature? How long ago was she in season? Any chance she could be pregnant? She is of an age when that uterus should come out if she is still intact and presumably not breeding! Here is why:
Pyometra, or pyo for short, is pus in the uterus.
Here's link.
http://www.thepetcenter.com/sur/pyo.html
Please take your girl's temp, and go to an emergency vet if she gets worse. Or at least see the vet in the early morning--call ahead today, then be there. Obviously, your dose of septra is too high for her.
