Help! My goose keeps falling over

I have a brown Chinese goose that keeps falling over.  She is 5 months old and this just started suddenly a few days ago.  Her legs seem a little swollen... and she will walk fine for a while, then fall over onto her back and is unable to right herself.  She is eating and drinking fine,  pooping fine also.  She will wander with the other 3 geese, but after a little while just falls over.  I’ve also noticed her wings are shaky when she stretches them out.  I recently changed their food over to a better pellet... All Flock.  I brought her in for a few days and gave her some electrolytes in water in case it was overheating.  But I can’t tell a difference.  I let her out again today with the others and have had to turn her upright a few times.... anyone have any suggestions of what is causing this in only one goose?  Treatments?  

It could be a disease but also something like lacking Vitamin B.

Try Spinach and add a Riboflavin booster from the Pharmacy,
it helped my in Law's fighting chicken often and gave them a good kick after a month or so.