It sounds like you may have a ganglion cyst. Ganglion cysts are benign cysts that grow off of the sheath of connective tissue surrounding the tendons in the wrist among other places. They are usually firm but slightly rubbery and may move up and down when you move the muscles in your hand or wrist. The exact cause of ganglion cysts is not entirely known, although they do seem to be associated with repetitive movements and some people may be genetically predisposed to them.
Generally ganglion cysts do not require treatment. Occasionally they continue to grow and can become uncomfortable or can have effects on smooth motion of the tendons of the wrist. However, in the absence of these or any other symptoms, there is no need to seek treatment. In fact, many ganglion cysts will spontaneously go away on their own if you give them some time. If the cyst does become very large, painful,or cosmetically undesirable you can talk to a hand surgeon
about having it removed. Unfortunately, after the surgery
there is a relatively high rate of the cyst coming right back, which is another reason to avoid surgery unless it is necessary.
Talk to your primary care doctor
if you begin to experience pain or if the cyst grows significantly.