Oh yes! I never thought of that! She could be pregnant! (are you sure she isn't?)
How old is your dog? Is she spayed? If she isn't spayed, has she recently been in heat?
A yellowish/whitish discharge coming from ALL of her nipples usually signifies either an actual pregnancy or a false pregnancy. Sometimes, due to hormonal fluctuations, even if a dog has not been bred, when their heat cycle ends their bodies think that they are pregnant and they will develop all of the signs of actually BEING pregnant.
If the discharge was only from one nipple then I would suspect some type of infection, but with it coming from ALL of them, the only thing I can come up with is pregnant, either actually pregnant or a false pregnancy.
Ghilly
I guess she hasn't been spayed? Nipple discharge can be caused by hormonal fluctuations, can be related to False Pregnancy. The discharge can be yellowish, brownish, or clear fluid.Sometimes it can be milky.
But on the other hand this symptom can be related to mammary tumors. Even benign ones can cause discharge.
It sounds as if it could possibly be hormonal, as she has the discharge in all her nipples at once.
I would say, to be on the safe side, you should check this with your vet.
If she HAS been spayed, then this is quite unusual, and would definitely warrant a vet visit.