Generally it means just having painful testicles from either too much sex or too little sex. There may be a more accurate description but this is what I always presumed it meant.

Or when you get sexually aroused but don't reach orgasm- it can cause pain in the testicles in men and cramps in women.

It's typically an excuse used by guys to get women to have sex with them. ALthough it can cause pain, it can be relieved by masturbation.