I had the same thing happen to me, scared the shit out of me, I even posted about it on here about a year ago. After some encouraging, I went to the doctor. After having my nuts fondled by several doctors, and fingers up my ass to check out my prostate, it turned out to be an infection in some tube that carries semen to your nut. Antibiotics ended up helping the situation.
Whatever you do, DO NOT let the doctor tell you its nothing. Insist on a second opinion if they tell you its nothing, and do every test possible until they find something. Nut pain IS NOT something to ignore.
Good luck man...
that shit hurts. :*( http://en.wikipedia.org/wiki/Epididymitis