There are a few things that it could be. It sounds like it's inside the scrotum, rather than on it's surface, leads me to believe its not an STD. Obviously being that there are a lot of blood vessels in that area it could be a vein or part of your reproductive organs. Best thing is to consult a doctor.
There could be any number of causes for a lump. If worried about a potentially cancerous growth, it would be attached to the testicle, but you should have a doctor look at anything just to be sure.
A lump above the testicle is something very serious and should be examined by a doctor immediatly. It is quite possible that it could be cancer which is what should concern you, however it could just be a cyst or a sac of fluid that will go away eventually. Either way you should seek medical attention.