Question about my female dog?

I just rescued a small female dog a couple of weeks ago. She is not spay and I am getting that done. I was checking her out today and noticed that her two back are a little swollen. I barely squeezed the nipple to check to see if thee was milk. It came out and was a real dark brown color. I am definately calling the vet to get her in first thing in the morning, but I am very concerned about her. Does anyone have vet experience to let me know if this means a mammary infection, cancer, or what could cause this swelling of two and dark drainage when squezed lightly. She is active and playful and eating good. Please help if you have vet knowledge.