I have read that application of medication to the inner side of the female privates will invalidate the fast. However, my question is if this ruling also applies to the female doctor’s fast too?
No, the fast of the female doctor remains intact. The ruling you have mentioned is covering two situations of the same scenario. I.e. any medication inserted in a female’s vaginal path will invalidate the patient’s fast. Only a Qadha’ will need to be kept and no Kaffarah.