My thumb locks up when in a certain position, what might that mean?

Trigger Finger. You've got a trigger finger (thumb). The easiest way to think of this is that the tendon is irritated and swollen so it can't slide normally as the finger moves. This causes it to catch. A cortisone shot is an excellent treatment -- probably the best non-surgery treatment. If the shot doesn't make it go away, then a small, easy surgery will fix it.
Trigger finger. Tendon gets inflammed and can't move freely. It may get better on its own but consult with your doctor. Antiinflammatories help.