What Causes Yellowish Discharge From Breast?

hi i am 21 years old. i have been having nipple discharge for the past 4 months. one breast produces yellowish substance and other produces white substance. but they are very little in amount. my periods are normal and i get them regularly. is my case normal or should i see a doctor?....i have no pain in breasts, no lumps or any other abnormal signs. thank you.