What Causes Popping In Knee Followed By Pain In Back And Legs?

I had a cervical fusion on my c-6 c-7 due to a car accident. Ever since my lower back and legs hurt when I walk. I'll be fine then when i start walking I feel a popping feeling and then the pain shoots down my right leg and I can barely put pressure on it. It shoots back up to my tail bone. It happens if I go for a walk, or go shopping. Shopping is the worst. IT started out small. Now its popping out of place every day. It use to happen if i walked along time. Now it happens if I walk at all for a short period. I have no pain when it doesn't seem to "pop" out. When it does i can hardly walk.