It's probably a ganglion cyst, perfectly benign. Usually if you stop doing what caused it, it will go away. For me, it was handsprings and pushups.
Do you sit for long hours typing? If so, I think you got carpal tunnel syndrome. Im not an expert, but i've had people at work who got the same lump. Some got it checked out and they had to wear a wrist guard. Research "carpal tunnel syndrome" online, it should be clear if you have it.
It sounds to me to be what is called a ganglion cyst or tendon cyst. These type of cysts are a small nodule under the skin that are moveable but feel hard. There is usually no pain with them they are just annoying. Dr's can remove them surgically or sometimes they will actually apply pressure to the area to break the cyst up. If it continues to get largers, non moveable, painful, or the skin around the area becomes red or hot to the touch then I would suggest going to the doctor as soon as possible. Otherwise it sounds like a very common, harmless tendon cyst. The reason that you may now have just noticed it is because the can lay deeper under the skin and come to the surface for no reason at all. .
STRANGE, 2 DAYS AND IT WENT AWAY AFTER RESTING IT !!!!! I THINK IT WAS FROM PUTTING SO MUCH PRESSURE ON THE BACK OF MY WATCH AFTER CHANGING BATTERY !!!!!