This type of fissure in the "gluteal crease" is relatively common. A variety of things can cause the skin to breakdown and crack, including trauma (too much scratching of an itch, over aggressive hygiene after a bowel movement, as examples), a fungal infection or dry skin.
If one has risk factors for sexually transmitted disease (multiple sexual partners, homosexual activity, previous history of STD's or any partner with risk factors) then immediate medical evaluation is indicated. If not then evaluation is also indicated if the problem has been present for a week or more. If none of the above applies, then it could be safe to attempt local care with a barrier salve (petroleum jelly or zinc oxide) for a week prior to seeing a doctor.Regards,