Nipple discharge can be because of a couple of things. A hormone imbalance can cause this. I had prolactemia myself in my early 20's. It's disconcerting. Since you haven't been sexually active, obviously this has nothing to do with pregnancy so thank you for adding that you are a virgin. When I had nipple discharge (also not pregnant), they did blood work first. I produced too much estrogen naturally. They also did an MRI. Our pituitary gland can have a small, harmless benign tumor in it that causes nipple discharge. I didn't have that so it was determined it was the hormone issue. It righted it self. If that is the case for you, they may put you on the pill. Do you have any pain or is your breast warm or anything like that? That would be indicate something like an infection but I doubt that. Is it one breast or both? And is it only when expressed verses just leaking out on its own? It is very true that usually nipple discharge is not serious. But do you have a doctor you can discuss this with?
You might want to read the info in the link below. It points out that discharge from both breasts is less concerning than if it were just from one breast, and other interesting information. Are you taking any medications, such as for allergies or anxiety?