The sharp pain
in the nipple specifically can be because of some sort of infection which may be deep sited and is not now visible from out side. Of this problem is only for once than it may be because of the muscle contaction, but if it is a continous pain than I would suggest you to keep an eye on it and have some sort of scan done to rule out any problem that can be there.
therapy for long duration can increase the chances of breast cancer
and I would suggest you to have screening for the same to rule out the breast cancer also.
Thanks and take care