There is every possibility she is pregnant, age is no barrier for either dog or bitch. The vet could have given her an injection to stop any pregnancy had she been taken within 48 hours but it is too late now. She could be aborted and spayed, but to be honest that is more dangerous and traumatic than letting her have the puppies.
No need to feed her any more until about the last week (nine weeks pregnancy) unless she is really thin. Good quality food, and split the meals up to three a day when she gets near whelping so she does not feel bloated.
Arrange to have her spayed when the pups have been weaned.