It could be that's she all tired out and would rather sleep

I wouldn't worry about a puppy skipping a meal here and there - they're fairly notorious for it.
When switching foods though, it's usually recommended that you mix the new food in with the old, gradually increasing the percentage of new food in the mix until eventually you're feeding the new food straight. The whole process usually takes about a week to ten days. This is to help avoid any tummy upsets from a cold-turkey food switch (which happens sometimes, especially if there's a big differential in the food quality).