It could be a split heat but because of the amount of lethargy and wobbliness, it does sound like a uterine infection (pyometra). Occasionally vets try antibiotics but the best treatment is an emergency spay. If this is in fact what it is, it is a very serious situation (maybe the vet thinks this case is mild though...). If it were my dog and the vet thought that it was pyometra, I would talk to them about spaying her today. I might also get a second opinion.
There are surgery pics on this site...don't know how much you like to see...:
Pyometra In The Dog, A Serious Uterine Infection Requiring Surgery... ThePetCenter.com