I hate it when people come into my clinic and ask for their dog to be tested for "poison." Invariably, it is not poison, it is some other problem. Besides, it costs about $1000 for a "poison panel" because each poison requires a separate test.
You say nothing about the age, social situation (free roaming, in fence) , etc of your pet. Without that, there is no way to speculate what kind of "poison" or problem your pet could have going on.
There is a great shirt on Cafe Press: If you cannot afford the vet, you cannot afford the pet. It applies here.