Nutro is in the process of changing packaging/names and also making some minor formula alterations... chances are what you were feeding is gone and has been replaced.
Diet food is not a good choice, period. Healthy weight loss is achieved by feeding a high-protein, moderate-fat food in reduced portions. Diet/lite foods tend to be low-protein, high-carb and too low-fat. Dogs on diet/lite foods are not getting their nutrient needs met, and slow their metabolisms down in response (sound familiar? "real lazy and just wants to sleep" and "she's been gaining weight"). If you want to feed commercial kibble dog food and are going to be shopping at Petsmart, I would suggest Wellness CORE Small Breed formula... MUCH higher protein, some higher fat, more nutrient-dense. This is a grainless food, and you will feed less... I would probably start with feeding 25% less food than you have been feeding, and weigh your dog every week or two on a scale that will do fractions... if she gains ANYTHING, cut her back a little more... if she loses anything, stay at that amount until she quits losing, and if she is still too heavy, cut her back a little, again.
If your dog continues to be lethargic and does not lose weight with very small portions of food, it is time for a vet check and a full Michigan State thyroid panel... this has more diagnostic value than a regular thyroid test.
It is not necessary to never give your dog treats, but most people give not only too many treats but too LARGE. For a toy dog, a bit of something the size of a lentil or a peppercorn is plenty. Treats need to be only a taste, not a bite!
ETA: Just read Jill S.'s response... bwahahahaha! I had no idea it was Comedy Night on Yahoo! Get a clue... how does an evolutionary diet not make more sense than little baked bits of starch and chemicals? I've been abusing dogs and cats with raw animals to eat for very many healthy years... they line up happily every night for even more tasty abuse LOL!