My 3 weeks old son has swelling in his right nipple, is that cause for concern? what should we do?

Usually normal: It's normal for babies (boys and girls) to have mild or even swollen, enlarged breasts and/or lumps under the nipple. Almost always benign and due to exposure to maternal hormones in the womb, same hormones that cause the mother's breasts to swell and produce milk. Over weeks to months, no more exposure to the hormones, the breast tissue begins to shrink and eventually flattens.