You should see a doctor and have this checked. It matters if the milk is coming from just one or both boobs and if it is coming with or without stimulation. If it is coming with stimulation, it could be due to overstimulating or other benign causes. It is more serious if the milk is coming without any stimulation.

There could be a number of reasons: an infection inside the breast (the discharge would appear in just one breast), high prolactin levels (that have other causes including benign brain tumor), breast cysts, etc

the only way to know what's going on, you need to be seen by a doc!

I hope everything turns out just fine. Good luck!