HELP! My dog won't eat and is starving to death!

I have a two year old Yorkie-Poo. We had her spayed 2 1/2 weeks ago and things went great. We had her in to remove the stitches last Friday, and since she has stopped eating. Friday things seemed fine. At the surgery she was 7.0 pounds. At her peak she has been 7.9 pounds. Yesterday she was 6.0 pounds. She is nothing but skin and bone. I can feel all her ribs, her entire spine, and her hip bones stick way out. Saturday morning she threw up and wouldn't eat anything. So I made her some hamburger and rice (mild, at my vet's recommendation). She ate a little and kept it down, then pooped a big white nasty pile later that day. But she didn't eat anything more all day Saturday or Sunday. Monday morning I went and got her some soft food. She ate an entire can (small size), and threw it up about a half hour later. She hasn't eaten anything since. She drinks water regular, pee's regular. The vet took x-rays to look for a blockage, and found nothing. temp was normal. Took blood and did a complete workup and things were mostly normal. Slightly elevated white cell count, low electrolytes (from vomiting), and slightly elevated kidney enzyme which vet says is just concentrated since she isn't eating, but nothing to worry about. With the white cell count, they put her on antibiotics thinking she may have a slight infection or inflammation. I don't know what to do and am worried my dog is going to die. She hasn't kept any food down in over 4 days, and she has no weight left to lose. My vet doesn't seem real concerned at all, but she needs to eat. Usually he has been a great vet, but I am considering taking her somewhere else. Anybody have any advice, tips, or experience with something like this. I don't want my dog to die.
I have no suggestions as to what might be wrong with her, but I can't help but think your dog needs to be on an IV drip. She didn't have a lot of weight to lose in the first place and after 4 days without holding down food or water (is she?) she is in serious danger. If you vet refuses to do antyhing maybe he doesnt believe how serious it is? And remember that the latter stages of this kind of situation can be more marked than earlier - maybe SEEING your dog now in her current state would make him more attentive?
That's what is so bothersome, my dog spent all day at the vet yesterday so tehy could monitor her, and she is there right now for the same reason. I asked yesterday about what to do since she is so skinny, and they agree that she is very skinny, but said "It's not a problem, dogs can go 1-2 weeks without eating anything with no problems at all." And even when I asked if that was true even in her emaciated condition, they said not to worry. She is getting weaker every day, that is easy to see. And the look on her face breaks your heart. She looks at me with her big brown eyes and you can see she doesn't feel good. But at only two years old, and they have done a lot of tests, I don't know what to do. I can't stand putting her to sleep without knowing if there is something very wrong with her, but the vet didn't find anything and seems unconcerned. To answer your question, the last thing she ate was Monday morning and she threw that up. She is drinking water regularly and peeing normally, but last night she threw up some water and yellow stuff (which was a high calorie formula the vet shot down her throat to get her some nutrients and calories), and this morning she took a big drink of water and threw it up too. Between those two vomits, she probably drank water another ten times and never threw up. Thank you for the reply and help. I am getting qutie worried and considering another vet. I thought an IV would be in order too, but the vet hasn't said anything about it yet