Hello and thank you for asking your question on just answer. My name isXXXXX will try to help.
Does the drainage have an odor?
That is not a normal finding after breeding or anytime.
The yellow indicates a possible infection.
She could have a vaginal infection, a uterine infection, or a urinary tract infection.
She needs to get started on antibiotics right away. I would take her to the vet today. The worst case scenario is she has a severely infected uterus which needs to be surgical removed. This mean she would have to be spayed to remove the infected uterus. Since she is trying to be a mommy, you do not want this to happen. The sooner you get her on antibiotics the better.