What Causes Stiffness Just Under Breasts?

Each night, 2-3 times I have a constricted feeling just under my breasts. The best way to describe it is the feeling you get when startled or when feeling anxious. There is no acid associated with it. It wakes me up and can last several seconds. I am taking meds for acid reflux but this issue has been going on for about 5 or 6 years.