What Causes Pain In Right Breast In Breastfeeding Mother?

Hi. I'm a mother of a six months infant. And I have been feeling pain in my right breast since past two days. I usually feed my child through right breast and now it pains so much that I can't feed him. I also lifted my brother's daughter with my right arm. So I'm not sure what can be the reason for this pain. I feel lumps in my breasts under my nipple. Please state what can be the cause of this pain ?