To me it sounds like a lack of shoulder and upper back flexibility - when you hit the heavier bag, your shoulder engagement is greater and therefore the impact requires greater flexibility to absorb. Start doing some good static stretching, 10-20 minutes a day, for this area. Hold each stretch for 40 seconds and do it 4-6 times. Usual Trapezius, Deltoid and Lat stretches. Meanwhile, try to get a doctor's appointment.