Hello My Left Breast Leaks A Very Oily Substance Once

Hello my left breast leaks a very oily substance once in a blue moon. Very random. It soaks my bra through to my shirt. There is no smell or color just extremely oily. It leaves my nipple sensitive. Ive been on the same birth control since i was 16 i am now 23. This just started happening a year maybe 2 years ago.