SweetPepper

It is quite likely a yeast infection. You probably sweat when you workout. Yeast thrives in moist, dark areas. You can buy any over the counter antifungal cream or spray powder. You can either look in feminine hygeine and use a cream for vaginal yeast infections, or you can look for something that treats jock itch or athletes foot. This is also common in overweight or obese persons and can occur anywhere skin overlaps skin, such as breasts, sides or under the stomach. The treatment may burn for a minute or two but that pain will subside and the healing will be worth the minute or two of nearly unbearable pain.
The best way to prevent the painful recurring infection is to keep your skin dry. It may be necessary to use powder or keep an absorbant cloth tucked into areas where skin overlaps. If needed, use cotton or qtips to dry your navel.