
I had one who, unlike the others up to her, came back into a more coloured (blood) discharge through the third week after two matings. But it didn't smell or look odd and she had an uneventful pregnancy and 9 super puppies, albeit by C.Section.
What you describe could suggest, to me, a pyometra. If the stud dog was a popular one and the owner didn't clean him well after his matings, it's highly possible for him to have introduced a bug in there, picked up from a previous bitch. And if, like mine, she's low to the ground and you take her off your property for exercise, that too could means she squats and picks up infection. I have never exercised our bitches off our properties over the years, when in season, and we've never had a pyo. Others will no doubt disagree, but I'm convinced this is relevant.
Hope your girl will be okay.