On a diet?

I am really stressing out these past couple of days. My dog Sundae hasn't been eating much. She just plays with her food and that's it. I don't get it. She's not sick or anything so why all of a sudden she lost her appetite. Well, I switched to a different dog food because her old one is now really expensive. The vet told me while I was buying pet supplies said that her new dog food is also good for puppies but is less expensive than her old one. At first she was able to know that I've mixed in a few scoops in her old dog food. I did the gradual weaning to the new mix and soon enough she was able to adjust. Just a week ago, she was enjoying the new mix but a couple of days ago, she barely touches it. I think it's time for a check up. I'm afraid she might be underweight already. :(