Any heavy use of the
pec even one lifting episode or pulling a heavy door or object away from U may have caused a strain & now it is sore.
Also a blow or minor contusion of the chest wall or again a pulling or lifting episode that goes unnoticed can cause underlying chodrochondritis, which may B the cause. If it the left pec area it could B cardiac in origin. Have your PCP eval ig 2 B on the safe side.