If it was a single episode,then it could be due to undue pressure on your left hand due to wrong sleeping posture ,resulting in swelling on your hand.
This swelling ultimately went away in morning,once you slept normally.
However if such episodes are repeatedly occurring,then it could be due to your uncontrolled diabetes
,fluid retention in body,congestive heart failure
In such case,you should consult your doctor and get examined and investigated properly.