There are a number of reasons, but colon cancer is highly unlikely. It could be something as simple as some inflammation in your gastrointestinal tract, but if you want a definitive answer you'll have to visit a doctor. Happens to me occasionally too, so don't freak out.
Ok its probably hemmoroids. It happens if you push too hard and a symptom is feeling like you gotta go and blood, its just swollen rectal tissue...then when you poop it cuts it and you bleed. Get some hemmoroid cream all stores have it, it will treat it and get rid of the burn.