Passing blood in stool is not something to be taken lightly. It might be something as simple as hemmeroids or could be a more serious matter. I would suggest you make an appointment with your doctor and see what they have to say. Best of luck to you.
While I agree that this is something that you should inform your doctor of, I'm thinking hemorrhoids or fissures which can be brought on by frequent diarrhea. Also being as young as you are is an extra added protection, but I would guess that your eating habits are that of young person as well. Try adding 25-35grams of fiber a day to your diet. This could be done by taking a fiber pill and drinking lots of water. YOu could also acheive this by adding a high fiber cereal, beans, wholewheat and lots of fruit and vegetables. Make note that this will cause an increase in gas at first, but will help with your diarrhea in the long run and should make you more regular.
if you are passing blood i would see a gastro/digestive specialest to make sure what it is you are dealing with .i don'think i would try to guess work this.as baceball mom said it could be something simple or it could be something more serious .go to see your doctor .good luck