Hygiene. If you haven't showered during the previous 18 hours, it's likely you'll have enough sweat, sebum and perhaps cotamination by semen or feces to have enough bacterial overgrowth to create a strong odor. The cure is better hygiene, perhaps looser-fitting underpants.
Fungus. I would definitely have a physician examine the region, but one of the common sources can be a fungal infection or 'jock itch' which can be relieved with a topical anti fungal like lotrimin (clotrimazole).