I would switch vets, yours doesn't seem to be thouough enough. She may be an old dog, but still deserves to be cured, if possible.
Was she ever spayed?
The reason I ask is to know whether this could be from her womb, but it would be difficult to tell, as any blood that comes out could be either from her bladder or vagina.
It is possible that she has bladder stones, or even a tumor in her bladder. I don't say that to upset you, but these things do happen sometimes.
However, the greenish-yellow discharge she had before sounds rather like a womb infection to me. But it can't be if she has been spayed.
I'm the sort of person who doesn't like to give up on an old dog. But the facts are that an old dog has a very very tough time with any kind of surgery. They CAN be ok, but it does depend on the kind of surgery, and the general fitness (for the age) of the dog. So it makes me wonder if the vet was treating this with antibiotics to see how that would work, because it is a non-invasive way of dealing with any infection (whether from womb, or bladder)
I think she does deserve an x-ray (if she would be ok with whatever sedation they would use) -just to have an idea what you might be dealing with. An ultrasound might help too?? It may even be possible to do an ultrasound without sedation, if the vet would allow you to be there to hold her very still and stop her panicking while they do it. It may be they will find nothing, and this is an infection. In which case, she will need another course of antibiotics, or a different antibiotic perhaps.
But I do feel that whatever the results of any tests, if they show something serious, it might be best not to do anything too invasive, and let her live out her days with peace and happiness and whatever palliative treatment the vet suggests....? It would be up to you.
I know, this is hard. My dog is like a little sister to me too.
Oh Sweetheart.......I don't know the answer, but want you to also post your Exact question over in the "Ask A Vet" forum....It will be under "General Health". This will give you an opportunity for a Doctor to answer it, too......Hang on, Some ideas will come in soon.....Karla
P.S. Is she Spayed????