Hello
Hot fermentation and then oil massage twice a day might help you, massage gently neck area, below the head and behind the neck.
After heat do neck and shoulder stretching n mobility exercises.
You can buy posture corrector belt for support it will help in pain relief.
Next Steps
Avoid neck bending and lifting weights, keep baby supported to avoid lifting.
Health Tips
prefer side lying position during breastfeeding, During sitting place pillow below baby while feeding to avoid neck bending.