
Unlike all the others, I had one bitch who after being mated, produced a blood-coloured discharge through the final weeks of her season. I was worried but since she was otherwise fine, I didn't get her checked. She had an otherwise uneventful pregnancy, and 9 really super puppies - it was probably one of the best litters we ever had. We kept 3, 2 bitches and one of the 5 males. Something I hardly ever did.
Personally rather than rush her to the vet (infection), I'd keep an eye on her and if she's otherwise okay, not drinking more or otherwise unwell (pregnancy apart) let nature take it's course. If the discharge becomes copious, or foul smelling (or the increased drinking) then of course, get your vet involved.