GliderMommy
I have found that melons tend to have that effect on my boys if they get too much or too often. (Watermelon, Cantaloupe, Honeydew, Etc...) Assuming that you have already been to the vet and all other avenues have been checked and covered... then it is probably diet. An easy fix for the most part.

Persephone
Has she seen a vet recently? You dont say how long you have had her or how old she is. If she hasnt, then she needs to as it could be something to be concerned with and perhaps contagious. Better safe than Sorry.