This can be a very common occurrence in certain conditions. I don't know if masto is one, but it makes sense. It is called Costochondritis.

From Mayo Clinic website:

Costochondritis (kos-toe-KHON-dri-tis) is an inflammation of the cartilage that connects a rib to the breastbone (sternum). It causes sharp pain in the costosternal joint — where your ribs and breastbone are joined by rubbery cartilage. Pain caused by costochondritis may mimic that of a heart attack or other heart conditions.

Your doctor might refer to costochondritis by other names, including chest wall pain, costosternal syndrome and costosternal chondrodynia. When the pain of costochondritis is accompanied by swelling, it's referred to as Tietze syndrome.

Most cases of costochondritis have no apparent cause. In these cases, treatment focuses on easing your pain while you wait for costochondritis to improve on its own. http://www.mayoclinic.com/health/costochondritis/DS00626

It can go away without treatment, though it is miserable while it is there. Sometimes they use NSAIDS or muscle relaxers.