I had a small breed poodle mix and he did the same thing. He would be really wobbly for awhile not being able to use his legs, we took him to the vet numerous times before they told us he had a liver shut (which is common in yorkies). Basically your baby may not be processing protein due to the shunt and it causes that to happen in her legs. You might want to mention that when you see the vet next time.
(He too would gain and lose the use of his legs in the matter of hours). Until then watch how much protein your baby is eating.