HI icosad,
The most common reason for sudden lack of appetite , vomiting, diarrhea and lethargy is an obstruction of the bowel. If the pups has eaten a non food item such as raw hide, bone, toys, stick etc. then you may be looking at a blockage. A blockage can come from any household item the pup may have eaten, it does not have to be what you know you have given him. I am going to direct you to a site for this and it will go over symptoms and explain what happens with an obstruction.
http://www.petplace.com/dogs/gastrointestinal-foreign-body-in-dogs/page1.aspx
Since she is up to date on shots then parvo can be ruled out. If she has gotten into the garbage at all of has eaten any fatty human food sucha s steak, ham gravy, etc. then this can cause pancreatitis. This would require a 24 to 72 hour fast with an IV drip to keep the dog hydrated.
http://www.petplace.com/dogs/acute-pancreatitis-in-dogs/page1.aspx
dog training consultant
24 years training & behavior,pet first aide, Therapy dog eval., PA Animal Response Team