What is this lump in the ?

m husband had a vasectomy about 4yrs ago , the other day he was complaining that one of his testicles throbbed....well three days later a lump has appeared in it is not actually on the testicle but seems to be attached to one of the tubes in the its about the size of a brazil nut and is firm to touch but extremley painful. Does anyone have an idea what this maybe ? im worrying !