Hello,
Thank you for asking your query.
What you are having is a condition called Gynaecomastia. The onset of your problem happened during puberty so this is most probably pubertial gynaecomastia. As you correctly said a small disc shaped mass should be palpable under the breast in true Gynaecomastia. But in real practice this may be difficult to appreciate with bare hands. So this is not necessary to diagnose gynaecomastia.
Generally, no treatment is required for physiologic gynecomastia. Pubertal gynecomastia resolves spontaneously within several weeks to 3 -5 years in majority. Also you should know that there is no harm caused by this swelling. Chances are that it will disappear in few years if you maintain an active lifestyle.
There are some drugs and surgical options, but it needed not be considered as of now.
Hope this helps.
Thank you