my son is 15yrs, he recently noticed a small nodule/lump like thing may be around a groundnut size under his left nipple. Its not visible externally, pains slightly when pressed, but its just few days and find it to have doubled but its with same pain symptoms (slight pain when pressed). what could that be, One more thing is he recently has got facination toward doing exercise and startd doing push-ups. can you please advice, what could that be.