Normally my dog is very energetic and she will always want to play. For some reason today she won't walk up stairs or jump on the couch or my bed like usual. She is a 2-year-old pit bull that is very small for her age. Does anyone know what could be wrong with her?

Honestly, there are too many things that could be wrong for us to guess.
She could be not feeling well in the stomach or she could have an injured back or leg... there is no way for us to know.

You will have to consult a vet on this one I'm afraid.