As kjc posted this is most likely due to an empty stomach. Ruby, my smallest has done this before and even Reno has a few times. Neither of them are real interested in having their breakfast early, but will always eat a few kibbles for me and this does prevent them getting sick. Also, I used to give them a small snack right before bed time and that seems to help too.
__________________  ~Ruby, Reno, Razz, & Jack~ |