What is the problem if i am spitting up yellow mucus before brushing your teeth?

You have an infectio: Yellow mucus is a sign of infection. It could be coming from your sinuses or somewhere in your mouth. You should see a doctor and have this checked out.

Pus...: It sounds like you may be discharging pus from an abscess. Usually an abscess originates from a bacterial infection. A pus taste may also develop. There are two types of dental abscess. A gum abscess and tooth abscess. Schedule an appointment with a dentist!