Can my employer demand a note specifically stating why my doctor has extended my time off after surgery?

I had a hysterectomy on March 17th. My doctor is not releasing me to go back to work until the 21st, and then only for half days until the 29th when I go back for my 6 week post op. I work for a doctor's office and the doctor at my workplace is demanding a note specifically explaining why I can't come back to work at 4 weeks fulltime. The hysterectomy was not an open abdominal incision just laproscopic, and yet they took all female reproductive organs. I want to know if I really have to have a note explaining this, or if legally I only have to provide a note from the doctor treating me saying when he releases me to come back to work, and what restrictions he is placing on me.