Based on what you have posted , differentials will be a superficial skin infection such as a boil or it could be a perianal abscess.
Have you been biking for quite sometime already?
Are you engaged in a job that requires you to sit for hours at most time of the day?
A boil is due to a bacterial infection and may start as a reddish bump over the skin. The bump may eventually enlarge and drain pus profusely. This may be associated with fever at one point and severe pain and tenderness over the area.
On the other hand, a perianal abscess may be due to an inflammation and infection of underlying structures of the rectum and anus :" It arises from an infection at one of the anal crypts of Morgani which leads to inflammation and abscess formation. "http://en.wikipedia.org/wiki/Perianal_abscess
You need further evaluation by a physician. An internist may be able to assess you completely and suggest for referrals if further intervention may be indicated. You may be needing oral antibiotics.
Keep the area clean and dry and wash only with mild and bland cleansers.
Have you been diagnosed with diabetes before?
Any other similar lesions in other parts of the body?
What kind of underwear did you wear? I run, and I've found that the special runner's underwear (with minimal seams) is much more comfortable and causes a lot less rubbing against the skin down there. I've heard MANY cyclists complain about perianal skin irritations, so you're not alone.
Generally, you get what you pay for with that stuff.