Yup, I agree, sounds like a ganglion cyst. Once you've had one they tend to come back - even after surgical removal. I get one the size of a golf ball on the top of my wrist.
Wearing a soft wrist brace or ACE wrap might help. Movement can irritate the cyst and make it last longer.
Like the previous poster said, do see a doctor, they can remove the whole cyst in an outpatient surgical procedure or aspirate (remove the fluid) in your doctor's office.
If it's not a cyst, it might be a benign bone tumor - also harmless! - it can be removed by a orthopediac hand surgeon. On the whole BBTs tend to feel harder and are more painful than ganglions, but not always.