How old is Bella? And the other biggy question is she neutered? Aside from the fact she might just like the new food, change is good, and she might be going through a growing spell. If all the other signs are good ie: eyes, teeth, pee/poop, energy, my vet told me and I was a very much newby to dogs, check those things I listed above, and if you can still feel there ribs and or back bones but not sharp, then they don't weight too much. Now with that said, if Bella quites drinking to the vet ASAP, that is the one qualifier I was given and I live by, I just love our vet, she is part of our family now we have been so many times, I was such a scardy cat
New food is fun, I like new stuff don't you? Good luck keep us posted