I have a quarter size ball of fluid or pus on my knee. It pop today and mix of blood and yellow stuff came out. What should I do. Dr is closed.

Abscess. You have an abscess of the skin around your knee. You need to see a doctor ASAP. Go to your local urgent care center. Many are open until 8 or 9pm at night. You may need antibiotics or a drainage procedure to heal this infection. Warm soaks with epsom salts may help. You can take ibuprofen for pain.