Why do i have a nipple that goes in and have a milky discharge?

Has that always been: an inverted nipple?. Some women just have inverted nipples that tend to become erect with stimulation. If the nipple has become inverted more recently - that needs medical assessment. You may have some galactorrhea. I see pre-term labor listed on conditions - but am assuming you are not pregnant at time you are asking this question. That can be checked as well.

Is this new for you?: There are so many variations in how nipples look. Be aware that a NEW inversion in a nipple that has always protruded can be a problem - even a warning sign of breast cancer. Milky nipple discharge does happen, and is usually not a sign of a serious problem. The catch here is the word "usually." ANY new changes should be checked by your doctor; always check if in doubt. LGromkoMD