Upper back pain during breast feeding months/years is mainly due to increased overall upperbody weight, poor postural habits, poor posture during feeding baby and weak upper back and para scapular muscles.
U need to work on reducing all above factors...
Next Steps
consult with a qualified physio who will not just treat ur symptoms, but educate u to avoid ths problem in future.
Health Tips
hot water fomentation on neck and back
sit with back support
follow good postural & ADL habits