What Causes Milk Discharge From The Breasts?

Ill try to give you as much details as I can to better help with the answer.

I am a 28 year old female. I have 3 kids the youngest being 4. I have also had a partial molar pregnacy in 98. I had got my tubes tied the same day my last child was born. I am curently on my period. Which they are always very light. Now with the question. My left breast has started to produce milk again. Do you know what can be causing this?