Swelling under your nipples, or a “breast”-like appearance to the chest, is common in young men as they go through puberty. This is called “gynecomastia,” and usually is due to the normal changes in hormones during puberty. Most of the time, this will go away on its own, but there are some medical problems (like thyroid disease) that can cause gynecomastia, so if you have concerns it’s probably a good idea to talk it over with your health care provider.

Read more here: https://youngmenshealthsite.org/guides/gynecomastia/