My 4 week old foster puppy started on wet puppy food&shots 2 days ago.What's wrong?

Today has fever-no other symptoms. He is eating and his poo seems normal. He doesn't really want to play. He is laying on the air vent. No drool or eye goop. I'm not sure what he was eating before we got him since we got him the day he got dropped off at the shelter. I am feeding him the puppy wet food the shelter told me to feed him. What's wrong? Should I take him to emergency vet or wait until tomorrow and see if he needs to go to my regular vet.