I have had this problem, but both nipples. It was caused by an elevation of the hormone prolactin. A blood test will determine if this is the cuase, but if it is only on one side I kind of feel funny even bringing this up. however, if it gets you to your doctor, then it will have been the right thing to do. Elevated prolactin is linked to stress, but also other things. Please, tell your doctor.
Godd luck... Miri