Catch in my left thumb

In the past month or so I've noticed a catch in my thumb.  Sometimes whenn I move our in a certain way it feels like it won't move any further Ehren though it should.  And maybe half the time I can bend it in a different way it will emit a audible crack and then it will move freely. It seems to be intermittent and is not painful when it happens - just mildly uncomfortable.  There's a good sense of relief and things feel like they are back to normal immediately after I hear the crack.