You said she had papers. My first thought was, "Isn't the breeder name on the papers?" If so, no problem there. If not, then I would definitely see about having your vet check her.
She IS cute though
And, I don't think 4.5 lbs at 5 months is too big. Mandie was just over 4 lbs at 5 months. She is a year now and right at 5.5 lbs