Hi!
More info? Did you have blood taken? or an insect bite? Is it inflammed, red or radiating warmth or a track line up your arm? Did it start out as the size of a pea? How fast does it seem to be getting bigger? Can you bend your arm? Is there a pulse in your wrist? Fingers ok? Does your hand look just like the other one? If you were my daughter, (can't give any medical advice except to have a dr. look at it), I'd vote scar tissue is a good quess possibility, or even a cyst, (which is just a fluid filled sac) and maybe suggest a warm soak, (warm water on a cloth wrapped in a towel) to see if that helped at all, and where is the numbness exactly? How bad is the pain? and is it worse at sometimes more than others? My biggest worry would be an infection, and have you ever had numbness in your arm before? My arms and wrists and legs get numb and tingly alot. 
and if your gut is telling you to worry, call your dr to be on the safe side.
hugs,
suetoo