It could be a ganglion
cyst pressing on superficial radial nerve
causing compressive neuropathy, hence the numbness
in thumb. It is a benign lesion, not requiring surgery usually. It can regress with intralesional injection of steroids
. The best person to give you an apt advise would be an orthopaedician. Kindly visit a doctor nearby or place your query in skin, muscle joint section of this site. Take care.