Since you heard a pop sound, there may have been a muscle or ligament injury. Other possibility is a hairline fracture. The swelling could also be a contusion or a hematoma (localized collection of blood). This may take a few days to subside. However, please get this evaluated once. Take care!
The medical advice given should not be considered a substitute for medical care provided by a doctor who can examine you. The advice may not be completely correct for you as the doctor cannot examine you and does not know your complete medical history. Hence this reply to your post should only be considered as a guiding line and you must consult your doctor at the earliest for your medical problem.
Hi Leon, I realize this is an older post, but I thought others might benefit from further information on this issue. I feel that because of the 3 week lapse, you could safely rule out nearly everything but a hairline fracture. That "little ball" you are feeling is as you assumed - calcium. That ball of calcium will slowly return to original (or close to) size during the healing process, this is known as "remodeling" (this could take years). I'm sure you're back to your normal activities by now. Best wishes! jd1963
Leon, I did want to know that you should still be very cautious for at least a few more weeks, as you are still susceptible to reinjure that fracture. If you must play football (I don't recommend it), get some protection - a motocross chest protector works pretty well. Be careful! jd1963