TeenHealthFX is glad that you brought this somewhat delicate matter to our attention. Bleeding from a "butt crack" can be caused by a number of things, from simple things like irritation from clothing leading to abrasions and bleeding, to things like hemorrhoids and other causes like fissures.
If your rectal area is itchy, it could be a contact dermatitis (which is basically a skin irritation) caused by a new detergent or new soap, or from clothes rubbing against skin repeatedly and causing irritation then bleeding.
If you are sexually active, another cause of an itchy rectalo area can be an STD, like Genital Herpes or Herpes Simplex II Virus (HSVII) and Syphilis. They all can cause sores, rashes and itching in the genital areas, which can then possibly bleed. They are both spread through skin-to-skin contact occurring during anal, oral, or vaginal sex. Look at What's Herpes? for a more in-depth information about the Herpes virus, its causes and symptoms.
Another possible cause of bleeding could be hemorrhoids. Hemorrhoids are varicose veins that occur in your rectum (think little old lady veins but on a much smaller scale) that lead to little pouches of skin that be either on the outside of your "butt crack" (external hemorrhoids) or just inside (internal hemorrhoids). Sometimes you can feel them when you wipe yourself, but usually you cannot. When they get irritated - which can happen with just normal bowel movement, but is more frequently associated with hard stools or more often straining during the bowel movement - these little bits of skin can bleed. You often can see blood on the toilet paper after you wipe, or sometimes little bits of blood in the bowel after a bowel movement. There are quite a few things your doctor can do to help minimize the pain and irritation of hemorrhoids.
Finally, another thing that it could be would be something like a fissure, which is like an extra little tunnel to the outside of the rectum. This is important to take care of, as it could lead to complications like infection.
As you can see, there are a number of reasons why you could be experiencing bleeding from your rectum. It is important that you get this checked out by a doctor to determine what exactly this is. If you do not feel comfortable telling your Mom or Dad, you might want to think about going to a teen health center or local Planned Parenthood who can see you without your parent's permission, confidentially and on a reduced or no cost basis. If you live in Northern New Jersey call the Adolescent/Young Adult Center for Health at (973) 971.6475.