Hey there! Your case seems like that of trigger points activated in your subscapularis muscle.
Please consult a orthopaedician if you have not done physical activities for a long time now as trigger points do not "usually" develop that ways. There are quite some chances of nerve issues developed due to orthopaedic issue. You need a thorough examination. When you are idle and a trigger point has developed, there would be "stiffness" unlike your present symptoms. Such trigger points are developed in a physically inactive or sedentary individual mostly.
Next Steps
please visit a orthopaedician or a physiotherapist to be assessed and examined properly in person.