What Causes Muscle Pain, Burning Spasms And Stiffness Leading To Walking Difficulty?

I have terrible muscle pain in my thighs and also pain, burning spasms in hip and lower back that travels down my leg to my knee. I have difficulty walking, my legs almost don't want to move because of the stiffness also. I can't lay on my sides because of pain the only relief that I get is if I lay on my back. What could this be? I do have some arthritis.