Hello, I am 14 years old. I recently found a lump under my left nipple, immediately going online to research because of the fact that I thought it was cancer or a tumor, but instead finding out that it was a part of puberty and having a high chance of going away. I wanted to go online and let others know that this will not cause death and the fact of wanting to help; which I hope it did. Here is some information below that explains an issue that you probably have as a young teen.
"One of these changes can be a condition called gynecomastia, when breast tissue enlarges. That small lump with tenderness beneath the nipple is a normal part of puberty. In fact, about half of all boys develop gynecomastia during puberty. It's usually temporary and can happen on just one side or both."