The pain you are describing can be due various reasons like some trauma to shoulder (fall on outstretched hand) or due to neck faulty posture (forward head posture or rounded shoulders). This can give rise to pain while trying to reach the back.
Next Steps
Try to do some posture correction exercises like chin tucks, shoulder press. Do some icing on outer part of shoulder if it's tender.
Health Tips
Avoid going beyond painful range of motion. If there is any nerve impingement then symptoms will worsen.