Huh that’s a hard one. Start icing it and limit how long your up on it . When your up limit it to 15 minutes at a time, then lay down or recline & use ice for 15 minutes. Boy it could be anything even coming from your back. Ask your doctor if you could do PT. Utube has some good exercises for the knee,
They did find my bakers cyst through ultrasound while looking for blood clots.