I obviously can't tell you exactly what's wrong, but it definitely sounds like she is unwell.
I think you should take her to the vet as soon as you can. Today if possible. She is very young, and they are vulnerable at this age.
This is a very critical age, as Ginger said. Even if her shots are current she can still get sick since she isn't yet old enough to have gotten her full antigen load. You need to get her to the vet ASAP as it could be something minor, or it could be the first signs of something devastating, like parvovirus. DO NOT WAIT. GET HER TO THE VET AS SOON AS THEY OPEN TODAY. Please post back and let us know how she is.