If the blood was bright red, it may be due to hemorrhoids. If you strained a lot, you may have suffered a small tear in your anus, nothing to worry about as it will heal okay on its own. See how your next few bowel movements are, and if the bleeding continues, talk to one of your parents about it. There are hereditary colon diseases which can appear at this age with bleeding polyps in the bowel. But I would think your parents would be aware of this. Plus, you need to check and see if there is a history of colon cancer in your family, and if there is, this needs to be addressed right away.
I would suggest that you seek medical advice since rectal bleeding is never normal. In your case the cause is likely to be minor because you are too young to be likely to suffer from the more serious diseases.
regards
Morecambe