Your hormones can be out of whack. If you have cyst in your uterous or you have endrometriotris (sp) it can fool your hormone in to thinking you are pregnant thus causing you to lactate.
I looked up online and saw that it could be either a thyroid problem, a tumor, or breast cancer.. but I just went to the doctor a month ago and had my thyroid checked (normal), and a few months before that was checked for breast cancer during a regular dr. visit. (normal also) I'm pretty scared about all this still... don't know what to think. Any other advice?
You need to see a doctor about this. It can occur for some medical reasons that can be serious.
I have no idea why but I do have a friend that this happens to for some unknown reason. She has however had 2 kids and nursed them both. It does not necessarily mean you are pregnant though.