Aww Im sorry your puppy isnt feeling well. I know what you mean about it being hard to switch their food gradually....even if you do mix it they just pick out the one they like anyways. Last week Bella went through something similar....only it was coming out the other end
.....I took her to the vet and he did say it was from change of food. He gave me some medicine to help her belly and I just continued to give her the new foods but went easy on treats and stuff. She seems fine now. Can you give her some nutrical, just because she threw up 3 times it may help her out a bit. Good Luck!