you could have a spermatocele. and accumulation of sperm in the epididymis. these may "come and go". less-common would be epididymitis but this should be painful. next time the lump appears make sure to have it examined while it is still there.
large lump on the base of the right testicle which comes and goes. over the last year i've had three ultrasounds and each time i went to get the scans the lump wasn't present. the ultrasounds did not detect anything apart from a minor varicocele. previous to the last 2 months the lump would appear randomly even when i wasn't engaged in any sexual activity yet every time i went to see my gp it wouldn't show itself. . recently i've noticed the large lump usually appears when i'm doing something sexual. thanks