The type of antibiotic to use would depend on the type of infection she might have going on. Often, a vet will use something in the 'cillin' family, but they may also choose to use a different antibiotic.
Another concern that I have is that this is a 9 year old, unspayed female who is recently out of heat and has bloody urine and is a bit lethargic. Because of these symptoms my biggest concern is that she's possible got a pyometra going on.
The term 'pyometra' means pus in the uterus. This pus formation in the uterus results from infection, hormone imbalance or mucous buildup inside the uterus. Most dogs and cats suffering from pyometra are presented because of loss of energy, bloody vaginal discharge, increased thirst and poor appetite, but not all the signs show up in all the dogs.
Pyometras can be either open (where the cervix is open and there is drainage) or closed (cervix is closed, no drainage, and dog is very sick). From the sounds of it, your girl could possibly have an open pyometra.
My suggestion is to get your girl to an emergency vet as soon as you can. If she does, in fact, have a pyometra and it is left untreated, it could very well rupture and kill her.
I hope this helps.