Difficult to say what it could be without seeing, but it could be a ganglion cyst
Thisis a tumor or swelling on top of a joint or the covering of a tendon. It looks like a spongy sac (cyst). Inside the cyst is a thick, sticky, clear, colorless, jellylike material. A physical exam is often needed to diagnose a ganglion cyst. See your doctor for evaluation & treatment options.