What Causes Pain In The Hip , Knee And Ankle?

Hello. I've sustained no injury, but for the past 4-5 months i've been experiencing pain (like a deep, boring pain) in my left hip, knee, and occasionally my ankle. The pain varies from mild to severe, sometimes accompanied by a "twisting" sensation in my knee. It's hard to distinguish at times if it is skeletal or muscular. What might this be, and what causes it?