My arms, mainly left one seems to go numb only when i'm lying my back. what could cause this? numbness is from shoulder blade to pinky and ring finger

Varies : The most common cause would probably be a pinched nerve in your neck. Other potential causes could potentially include thoracic outlet syndrome or one of multiple peripheral nerve entrapment syndromes such as carpal tunnel syndrome. I would not hesitate to be seen and a good exam could go a long way in determining the cause.

? pinched nerve: Although it usually occurs at a later age, these symptoms might be due to compression of nerve roots exiting the spine and serving the arms. A neurologist could clarify this with a careful history, physical exam and MRI of the spine.