What you have is Onychoschizia: splitting of the nails. Breast feeding is very demanding on the body of a woman. All the nutrients that are eaten by the woman are finally secreted in the breast milk. I suggest you visit a dermatologist since you are still feeding your baby. Especially since you have pain, you will need to be examined in detail to rule out other possibilities like paronychia. Not to forget that the medicines will be prescribe to you taking into consideration your breast feeding status. Plus a diet consult would be useful. What you eat and when you eat is very important. A dietitian will help you understand the science behind what you eat so that you and your baby both get the necessary nutrients in the optimum quantities.
Next Steps
Visit a dermatologist near you
Health Tips
Do not use creams or 'treatments' suggested by friends or family or pharmacists