This could be a serious emergency if your dog's gums are pale or "whitish"!
Recently my son's beagle developed these kind of symptoms along with lethargy.
She was near death by the time we got her to the Vet...they can go down that fast!
Sammy was diagnosed with IMHA (Immune Mediated Hemolytic Anemia) and it is a very grave illness!
I don't want to scare you but...PLEASE get your baby into the Vet's immediately even if you have to go tonight!