All males (and their evil sisters) are familiar with the crippling, delayed pain that accompanies a blow to the testicles. For just a second following impact, it feels like you may have dodged the bullet. Then, the growing, leg-weakening awfulness sets in.
Usually, the pain passes and everything's OK. You pick yourself up, dust yourself off, and look for your high-kicking sister, who has wisely vacated the area. But sometimes, taking such a direct shot to the testicles can cause other problems.
The testicles are each surrounded by a multi-layered sac, and a direct blow can cause blood to collect between these protective layers. This is called a hematocele.
A hematocele may simply be cured with bed rest and taking it easy, or it may require drainage during minor, same-day surgery. It depends on the level of discomfort, as well as the source of the bleeding that caused the hematocele. If the source isn't an external blow, your doctor will be hunting for another cause, in case it's something serious that's leading to the pain.