My bitch has been mated last week. Mated day 16 of her season she's now day 20/21 of her season, tonight I have noticed the blood is like a dark brown type colour. I have not noticed this change before in her seasons I am not sure if this is normal for a bitch coming out of season? Or if its related to her mating or possibly a infection?

She's been sick tonight but it was a hairball type sick. She's eating fine, drinking fine (not to much/not to little) doesn't seem anything off really.
Thanks.

It may just be old blood, keep a close eye on her,any hint of the discharge being smelly or her being unwell then take her to the vets.