Why do my breast produce milk without being pregnant and pregnancy test is negative?

Hormone imbalance. You're producing a hormone, prolactin I believe, that cause lactation. I would book an appointment with your doctor to get a hormone test, more then likely it is harmless, but you should get checked out to make sure that it is.

Please get it checked out. Craven is right in that an increase in prolactin levels causes lactation (breast milk), but in my case it was breast cancer at a very young age. Not meaning to scare you, but just pointing out the importance of medical intervention.