Foamy, yellow poop can be a sign of EYP/bacterial infection, worms, or kidney failure.
Taking a fecal sample to a vet (any old vet will do - just say you need the test results, not treatment) for a float test would rule out any kind of parasite infestation.
Blood work would be ideal... just say you need a diagnosis, not treatment. Most vets will be willing to help you.
I can't comment on cost, but I hope they wouldn't be ridiculous about it (maybe even do it for free!).
I am very sorry for your troubles... Is she still drinking?
If I were you, I'd move her to a crate in your bathroom or a garage - keep it dim and warm for her... Monitor her water intake. If she stops drinking, you will need to tube fluids to keep her hydrated through treatment.
MrsB