It also referred to as " bowlers thumb ", if you get it too bad your entire thumb can go numb. Its generally caused by your thumb hole being to small or squeezing the ball too hard. In some cases it can also be caused by how your thumb comes out of the hole and it can often help to add more bevel or have your span checked.
Walter Ray Williams JR. had issues with it due to some nerve damage from another injury, and he solved it by beveling the hole out in a certain way so that nerve that runs down your thumb doesn't get pinched.
I've had it myself since coming back from bowling, and mine was due to needing to buff out my thumb hole and squezing too hard. Its been several weeks and I still have a touch of numbness at the right tip of my thumb. Some of it though is due to an old baseball injury that can flair up occasionally.
