Slinging produces pinching pain under right shoulder blade

Any advice on how to improve my technique for it to not be painful? Im experienced with physical exercise and know to warm up and stretch, have rest days and stop when there is pain, however I only experience pain after slinging, not during, or else I would stop. I haven’t slung in about a week now since I have the flu and am trying to recover my shoulder. Im doing a full power throw for distance in the clip, but those are only about 20-30% of a slinging session for me. Rest is target practice or goofing off. Thanks in advance for criticism and insight!