what is causing stiffness/ache in hand but only at night/morning?!

I can not figure out whats going on. I have been waking up at night and in the morning with one hand that is achey and somewhat stiff maybe a little numb, feels a bit sprained but all of these symptoms go away about 30 minutes after I wake up? This is only happening in one hand and its not being caused by sleeping on the hand.

Any idea what is causing this? Im a little concerned as its been over two weeks now. It started out happening in just one finger and now its my whole hand/wrist.

thanks!