Could be a fisher (sp?) which is a tear in the wall and requires surgery to fix. These are extremely painful and can bleed a lot. You could also have an internal roid that has flared up. Either way, you need to see your doctor right away as you are way too young to be having that kind of pain...
Colon cancer generally occurs at agte 40-50+. It is extremely rare for it to be found in someone aged 16. However, rectal bleeding is never normal and should always be medically investigated - so do seek advice from your doctor. Could the pain be caused by constpiated motions stretching the anus? Maybe a high fibre diet and drinking ~ 2 litres of water per day will help modify your motions so that their passage is less painful.
regards
Morecambe
The water in the toilet turns red, and the tissue i use has blood on it. It hurts so much. Everytime.