Who fixes torn pectoral muscles over implants?

To answer your last question, the specialty to best advise you on the pectoralis muscle over implants is Plastic Surgery.  Based on your symptoms and description, I am not totally sure whether or not a secondary surgery would help, but I do think that the first person who would best know what was done to your pectoralis muscle would be your PS who put in your implants.  

I think that you may want to confer with an Orthopedic Surgeon to verify that the muscle is torn.  It would also be helpful to see some before and after photographs to assess.