Can a dog get mastitis after having been spayed?

She was spayed 6 days ago and I was routinely checking her sutures when I noticed blood. The incision site was fine but 1 teat had blood on it. I squeezed it and more blood came out. She has a normal temp and it's not swollen or painful. The emergency vet said if there's no temperature then she should be fine. She was not in heat or even close to starting when she was spayed After researching mastitis I'm concerned it could become bad really fast.