OK I gather she hasn't been spayed?
That can be hormonal changes which cause the nipple discharge. Mine used to sometimes get that happening, and occasionally even a fluid that looked very much like milk.
How long ago did she have her heat cycle? If it's close to that time or even up to 10 weeks or so after, it probably is one of the signs of what is known as "false pregnancy"
In itself false pregnancy is not dangerous. However it does mean that the hormones are swinging wildly, and it can pre-dispose the dog to developing Pyometra later in life (or indeed at any time. No age of female is immune from developing Pyometra!)
Of course, if you can feel any lumps in her breasts, then that can also be a cause of nipple discharge. However don't be terrified if you DO find a lump. Just go get it checked out as soon as possible. A large percentage of breast lumps in dogs are harmless. But it is wise to get them checked a.s.a.p.
Also....as you ask if she could possibly be pregnant....do you think she did actually mate with a dog while she was in heat? If so, then yes, she could well be pregnant!
Obviously if you are unsure, then get the vet's advice.
Agree with ginger. If you think she might be pregnant, you'd best take her into the Vet.
Actually, a discharge is not necessarily a bad sign but I question the color you are describing and that would have me at the Vet's office getting it checked out. There is no way for any of us to be able to tell you if she is pregnant. There would be signs but it could also be a false pregnancy. Do you see any type of vaginal discharge, like a straw-colored discharge? Is she eating or off her food?
Either way, I would suggest you take her in to be checked for pregnancy and that nipple discharge.
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