What Causes Stoppage In Menstruation & Discharge In The Breasts?

I haven't had my period for a few months, I took a pregnancy test after I missed my first period and it was negative. Anyways my question is that a few days ago I started having clear slightly milky discharge come out of both of my breasts and today more than a tiny amount came out, I barely touched my breasts and a few big drops leaked out...what could this be?