Extra breast tissue like a lump (less than 1-1/2 inches across) located directly under the nipple in teenagers is known as gynecomastia.
Tender breast are also experienced.
This, usually, happens due to male hormonal changes, known as physiological gynecomastia, and often happens during puberty and adolescence.
In some cases, it is related to certain diseases like thyroid gland disorders, kidney and liver diseases, or it is caused due to use of certain drugs.

It is well known that use of cannabis, can lead to gynecomastia.

This extra breast tissue, rarely, means something serious, but, is good to be checked by your physician for possible underlying conditions.

Best wishes!
Marija