Why is my long haired chihuahua losing her hair?

She had a puppy February 28th and has since seen the vet. She got a clean bill of health from the vet. She was not losing her hair at the time of the vet appointment. She started Sentinel on April 28th. No changes in dog food. One of my other dogs had puppies 6 weeks ago. I am not sure if this has effected her.