Bleeding from the back passage is actually one of the most common complaints seen by doctors. The good news is that most of the time, the bleeding is caused by something non-worrying.
For example, most people who experience bleeding, as well as a bump around their butt crack may have a haemorrhoid (your doctor can easily confirm this during his examination).
The appearance, colour and smell of the blood can sometimes help doctors distinguish between causes. As a general rule:
1. Bleeding from the anus – the blood tends to be bright red and fresh. You may notice blood after passing faeces, or streaks of blood covering faeces.
2. Bleeding from the colon/stomach/intestines tends to be darker red and mixed in with your faeces, because the blood has to travel a longer way through the gut before it passes out. This is known as "melaena", and tends to be more smelly.
You can find out more about what to do if you notice bleeding from your back passage in this previously answered question.