You might like to visit our Thoracic Outlet forum {TOS}-
it can be caused by MVA, desk/phone/paperwork, overhead or repetitive work, and there's many other causes also.
Posture - head forward/shoulders rolled or hunched forward?
-that closes the area where the blood flow and nerves travel down to the arm & hand.

Here's the useful sticky thread post # 1 has many links to medical sites that explain TOS.
http://neurotalk.psychcentral.com/thread84.html