My dog is suddenly acting lethargic and not eating?

It started yesterday. She's drinking water and doesn't have blood in the stool and no diarrhea. Everything is normal as far as feces. Sometimes she eats if I feed it to her, but this morning she didn't. She is a small black dog with long fur. I thought it might be depression, but nothings changed. I also considered heatstroke, but there aren't enough symptoms that match. Any ideas? She's usually a hyper-active-spaz, but now she doesn't run or jump. She just walks around like a little granny(and she's only 2 years). We also took her to the vet but they don't know what's wrong. They said to take her home and see if it clears up within the next couple days or they can do a whole bunch of tests to locate the problem further. But that's going to cost us over $1000.00 and we don't have that kind of cash. So we're making that option plan B.