There are two concerns that come to mind. The first is that she is experiencing a pseudo (false) pregnancy and you are seeing swelling of the breasts and discharge as a result.
My second concern is that she has a mammary tumor.
No matter what, you need to see the vet to have them take a look. This could be something as simple as an infected gland but you won't know until you see the vet.
If they do find evidence of a tumor, they will do a fine needle aspiration and remove some cells for testing.
I hope this helps!
Let me know if you have more questions.