Swelling between the testicles and anus may be the result of an infection. It could also be a blood boil, a simple blister or a cyst.
A hair follicle infection could be the cause of discomfort. There are many hair follicles in the pubic region that can become aggravated from sitting for long periods. This can lead to a blood boil. Your trousers chafing your skin can also cause swelling. A cyst can develop anywhere in the body, according to Medicine Net.
If this swelling is a recurring problem or has lasted for more than a few weeks you should seek a medical opinion. You may need antibiotics to fight a simple infection or you could have a more serious health problem.
Use aloe, petroleum jelly or moisturising lotion in that area as soon as you feel irritation or swelling. Medicated powders can help as well. They have antiseptic agents that cleanse, but avoid using too much powder too often.