At your age, it's probably hemorrhoids, especially if it's bright red blood. But any blood in the stool should be checked out. Fortunately, if you don't wait you can head off something that could become serious if left alone. If you are straining to have a BM, I'm sure it's hemorrhoids, but get it checked out just to be on the safe side.
Just set up your appt with a dr and talk to them. I am 33 been having it for almost 2 years now and have been so frightened about going but I am setting my appt today.. Don't wait just get it checked.. Good luck.