I had same thing when I was around 18-19. It's basically because of harmone development that your body goes through when you are "coming" into age. It's very sensitive, to the extent that you can not touch it (it pains upon touching).
If it is really big or growing, it may be Gynecomastia (http://www.webmd.com/hw/mens_conditions/tp21165.asp
) and it's better to get once checked by doc.
Finally, don't touch it or it will stay there for longer duration.
What is gynecomastia?
Gynecomastia Click here to see an illustration. is overdevelopment of the male breast. The glandular tissue of the breast swells, usually in response to an excess of the female hormone estrogen or a lack of testosterone, a male hormone. It occurs in babies, adolescent boys, and older men.
What causes gynecomastia?......