Vets on average do not research food and are not real experts on it so IMO never take a vets advice on dogfood. Most of the time they are pushing Science Diet or such and these are proven bad foods.
First when you change the food from Purina (not a good food at all) you have to do it very carefully especially with her size. I would definately change the food and there are tons of better food. You will need to do a gradual change where you mix the new food with the old food over a period of time (about 3 weeks is usually a good time frame) Mix a 1/4 of new to a regular bowl of old, then 1/2 and so on...
I would also give her nutrical (you can buy this at the vets office or a PetsMart/Petco. I would give it in the morning, middle of the day and right before bed especially during a dog food change. Also keep water and food for her right now even during the night. Limit her exercise and activity and do not let her over exert herself. Watch for signs of Hypoglycemia (lethargy, not wanting to move or play, refusing to eat, just not acting herself) if this happens then give Nutrical and get her to the vet ASAP. If you do not have Nutrical on hand and you have this type of emergency then give some honey or syrup. Just apply it to the tip of your finger and put it in her mouth and even rub it on her gums if she doesnt want to lick it.
As for finding a good puppy food, you can go to dogfoodanalysis.com and check the reviews. They rate the foods on a 6 star basis, 6 stars being the better foods. It also gives you ingredients and tells you why one thing is bad over another. This lady has put a lot of work into research of dog food. I am sure you will appreciate the link if you are picky