My Thumb Has Been Sore For About Three Weeks. I

my thumb has been sore for about three weeks. I have no idea why. the only thing I can think of is that I opened a jar with a very tight lid. If that is the cause of the problem It did not begin to hurt immediately. I woke up in the middle of the night maybe several days later and I could not straighten completely. The joint in the middle of the thumb was slightly bent and somewhat locked. It continues to hurt and is swollen. If I straighten it is painful and locks in place until I force a slight bend in it again. It is very sore at the joint where it joins my hand. I am is sales/design and only intermittently and briefly write and type so I don't think anything at work could have caused it.