if this is just a one off occurence you need not worry. You are very young to be worrying about colon cancer, though understandable to worry if your grandmother had it. Could what you describe be due to something you ate (ie. something other than blood)? If the blood is recurring or large amount then you need to see a gastro doctor to have it checked out. BUT - before you get worried, even if you do see blood again, remember that there are many many explanations other than cancer for blood in BM.
Go see your doctor. You don't have colon cancer, but should be checked for other things such as fissure or hemeroids.
Thankyou for your comments. Really has helped me a little.