For the last several months when I lay down at night I get pain directly underneath my sternum when I take a deep breath I am a 52 yr. Old female

As . As a rule of thumb i feel that all patients with retrosternal pain (pain behind the sternum) should have their cardiovascular system evaluated before considering the problem orthopedic related. Another possibility is gerd gastro esophageal reflux disease. Check with your family doctor to get basic screening for this. Only after these basic things have been ruled out should you consider the condition of musculoskeletal origin.