Breastfeeding, my nipple have yellowish pimpils, is it an infection?

Concerning: These may represent milk ducts that are full but concerning would be a skin infection or worse yet, mastitis. This would be most concerning if in addition to the pimples you have redness, pain, swelling, fever, joint aches and overall feeling bad. You need to call your doctor or lactation consultant to discuss your physical findings in detail and if needed go in for an exam and be treated.