Just so this may help anyone else looking at my question who might have a dog with similar symptoms...
We took my dog to the emergency vet because he started vomiting all night long. We requested that the vet perform bloodwork (that I thought at first he would not need because maybe he just ate something that upset his stomach.) After being lethargic for a few days and progressively getting worse and worse...I felt it was necessary to rule out anything serious. I tried to keep him hydrated overnight (because of his frequent vomiting) with a medicine syringe with water in it. I let the water trickle down his throat so he could swallow it and even though he kept throwing it back up 20 minutes later, I still felt like I was helping at least keep him somewhat hydrated.
The bloodwork confirmed what I was most worried about...he had severe kidney failure. His creantine level was a 6.5 so they put him quickly on IV fluids and he has been there since. Our vet thinks he will probably be able to make a full recovery and go home tomorrow with a special diet. This was really scary for us. He is like a kid to my husband and I.
Now I understand what serious symptoms to look for if a dog has eaten something toxic. Looking back on Monday, I now realize that his arched back while giving him a bath was his first sign. On Tuesday he was looking weak and walking around kind of spaced out and vomited once. Wednesday he was acting very lethargic, but still drinking water and eating the boiled chicken and rice I gave him. Thursday he was still acting lethargic, but ate some more boiled chicken and rice and then vomited a few hours later and then started vomiting more and more frequent. He kept me up all night Thursday night vomiting and refused water, food, and even nutrical. Friday morning we took him to the vet first thing and we are so glad we did when we did because if not he probably would not have made it. So he has been at the vet's office over the weekend on the IV fluids flushing his kidneys out and tomorrow (Monday) he will have his bloodwork tested again and hopefully will be able to come home.
Our vet thinks it could have been anything toxic like a grape or raisin (which I know we do not have at our house) or something he found in our backyard that was extremely toxic. He is the type of dog that gets into things and eats everything so I am surprised and feel he's very lucky that it took this long for something like this to happen to him. Despite our efforts to keep everything up high from him, he still manages to surprise us and climb or do whatever it takes to get to something he knows he shouldn't have. We're talking about a 10lb teeny Maltese here too... he was climbing kiddie gates and knocking big trash cans over as a puppy! He is so smart, he even outsmarts our efforts to keep things away from him sometimes.
I definitely will not take these signs lightly from here on out. I am very thankful that it was found in time and I'm eating my words above about not wanting to waste money on bloodwork. Thanks for your help! I hope this helps anyone else as well who might have a lethargic animal with the same symptoms. Our poor animals have no way of telling us what is wrong, so we must learn how to assess their serious symptoms so we know when its okay to treat at home or what serious signs we should recognize immediately to know when they need to see a vet.