Every Morning When I Wake Up, My Left Thumb Will

Every morning when I wake up, my left thumb will not bend. I have to try a couple of times before eventually snapping it, basically "cracking the knuckle". It is only my left thumb, in the morning and also throughout the day. It never gets "stuck" while using it. Is been like this for a very long time, but since it causes lots of pain. The Dr. give me a shot of cortisone about a mount ago, and was no changes. I'm starting to get concerned that with time it will be bigger problem (I'm a diabetic and has a deficiency of V b12.
What cause this problem? Please there are suggestions or advice?