Just want to tell you how wonderful your above response is! Kat So sweet, and so good!
go to the vet, we're not an animal forum, lol. is she pregnant
I can imagine how worried you are, but it's unlikely she(?) is in any immediate danger, so take a few slow, deep breaths, then pet her and hug her and let her lick away your tears if you are crying. (I guess it's obvious that I'm a fellow dog lover!)
Dogs can have breast infections, and after heat or a false pregnancy, a yellowish, or sometimes milky, fluid can be expressed. I imagine some of it could also leak out spontaneously. (Older dogs can develop breast cancer, but don't jump right to that conclusion, especially if you haven't detected any hard lumps.)
You could post your question in the Veterinary Expert Forum for a professional reply, but I think your best bet would be to make an appointment with a veterinarian, for a hands-on examination for her, and information and (I hope) reassurance for you.
Please post again and let me know what you've found out.
Best wishes!