What Causes Painful Muscle Spasms In Chest And Back?

I have had back/chest muscle spasms for the past week, it only seems to happen when i sit to relax or when in bed, it is very painful and makes sleeping difficult, however once I get up and move about the pains subside. get worse in the evenings and wakes me between 3 and 5 in the mornings. I have tried pain relief and sleeping in different positions but this hasn't worked. Any ideas please?