Sounds like you need to get her back to the Vet. Nicoal is right about the low blood sugar, for one. Did your vet do any blood work?
How old is your yorkie? These little guys are really prone to low blood sugar. You may want to offer small amounts of canned food through out the day. You can try white rice (like minute rice) and baked chicken to entice her to eat. You want this to be bland, so no seasoning or skin on the chicken.
Something else that you may want to watch for is a pyometra, an infection of the uterus since she's not spayed. The most obvious symptom is discharge from the vulva, but there doesn't have to be. I would Google "pyometra" to go over symptoms.
Good luck, hope she feels better soon.
Nicoal