Sounds like one to me. If it gets larger or it starts to hurt, you might have to see a hand doc (if you don't trust your PCP) and have it drained. My husband had a HUGE one on the side of his wrist that he had to have drained twice, but then after that it never came back. I've had cysts all over my hands, wrists and finger joints - I've had about 3 drained and 6 corticosteroid injections...fun fun...
But most times they will go away on their own.
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