Rectal bleeding is always extremely worrying, and can have various causes.
Mild bleeding is not uncommon, I have been told, but an appointment should always be made with your GP, who may examine you in the surgery, or refer you to your local hospital to find the causes, which can range from polyps, colitis, ulcers, hemorrhoids to other conditions.
Sometimes people may find bright blood after a bowel movement; this may be caused by haemorrhoids or an anal fissure. Blood in stools is something I have experienced myself. I have been to my doctor's surgery several times and also had a sigmoidoscopy, where a device is inserted into the rectum. I must add here, that I did not find it a painful experience, just a little uncomfortable.
Also some people may experience bleeding after passing a very hard stool during a time of constipation, due to a slight tear in the skin around the anus. Some infections can also cause bloody diarrhea. I experienced severe colitis many years ago, with accompanying liquid bloody diarrhea; it felt extremely acidic and my anus felt raw. It took me about three weeks to recover.
This kind of bleeding does not always come from the rectum however, so if you are seeing blood mixed with feces or the stools are extremely dark this may mean that the bleeding is coming from further up in the intestines or stomach, the red blood being altered in color as it mixes with stomach acids.
It is important to realize that there are many causes and not to self diagnose, but also not to assume the worse. Mild bleeding and blood in stools is apparently fairly common and the various causes of it can likewise be treated in various ways.