Pain under the left shoulder blade is usually a referred pain
from abdomen like stomach
ulcers, acidity, pancreatitis, or ectopic pregnancy (unlikely) or from the chest like in heart attack, pericarditis, pleuritis, pneumonia, pulmonary embolus, and aortic dissection.
Another possibility is ankylosing spondylitis. Back pain worse when sitting or lying down and feels better with movement is usually caused by ankylosing spondylitis. This is a genetic condition.