What could cause muscluar pain in right middle back? i lost weight and i was very obese and then it started aching. any idea?

Several answers: People who lose a great deal of weight, even by sensible dieting, often suffer gallstones, which in turn can cause right mid-back pain. Other likely causes are simply physical deconditioning due prolonged lack of exercise while obese. Poor body mechanics is another common causes. On occasion disorders of lung or pleura can cause such symptoms.