What Causes Popping Sound In Knee Along With Stiffness Behind The Knee?

My knee has made a popping sound while walking for years. 5 years ago I started working with preschool children, so I m in the floor a lot. I side sit, which has caused that knee to have pain in it. About a week ago, it started having pain around the knee cap, worse after sitting in any kind of chair, especially sharp while climbing stairs. It also hurts in my right butt cheek. Now, for 2days I ve got pain in my coccyx, and last night it was stiff and sore behind my knee. Is this all related? And what can I do to relieve it? A previous Dr said I had arthritis that was causing the popping.