nipple discharge and tenderness.

For the past 2 months or so I've had a thick white discharge from both nipples, and they've been tender or sore.  At first I attributed it to my period, however that has come and gone twice with no change in the nipples. I even tested twice for pregnancy to be sure that wasn't it.  I have a doctors appointment in 2 weeks, just curious if anyone else has had this issue or knows what it could be?