Could be a number of things. If you have overall weakness in the shoulder, thats a sign of a cuff tear. My guess from what you said is that its "weightlifter's shoulder" If you have a heavy routine, been working out for awhile and not giving enough time between workouts it will just become more painful. Touch your opposite shoulder with your hand. If it hurts on this "crossover movement" then its the distal clavical.
Give more info on how it hurts, where it hurts and what motions would give light to your condition. Best to see a dr. if it continues. Don't bother with a regular dr or ortho, see a shoulder ortho specialist.