I sometimes feel a burning sensation in the middle of back and across bottom of shoulder blades when i lay down. what could be the cause?

Varies: There are multiple potential causes, I would not hesitate to be seen for an accurate diagnosis. Considerations can include a thoracic disc herniation with irritation of a thoracic nerve in the spine, upper gi problems can sometimes cause pain in a recumbent position, subscapular bursitis can cause pain in a similar region. A detailed exam can go a long way in making the diagnosis.