Hi Jeri,
The one thing that comes to mind is that Krissy may have a thyroid problem . If this is the case, then she could put on weight easily.
In the picture she looks "pregnant" . My Suzi is 7.5 lbs, and is only 5 1/2 months . She does not look pudgy or bloated though. Did the vet check her sugar levels? If Krissy is having sudden spikes in blood sugar levels this could explain why she falls over after eating .