Hey,
I have a similar lump (age 22) and have had it checked previously. Apparently, in my case at least, it's a piece of cartilidge which loosely connects the ribcage. I apparently mananged to disconnect mine (playing rugby, fool) and it's never quite healed properly. Hurts quite badly sometimes but can be avoided by making sure i sit with good posture - plenty of swimming apparenlty helps as it builds up back and chest strength and gives a better posture.
That said - if in doubt, get checked out - never hurt anyone!
Hope that helps.