There is always concern when there is blood in your stool. The good thing is at your age you can head off anything before it gets serious. See a GI doctor and go from there. Good luck!
You need to see a gastroenterologist. It sounds like you might have an inflammatory bowel disease like ulcerative colitis. You need to get a diagnosis and treatment and treat any anemia you may have.
Are you having any abdominal pain, or pain when you go to the bathroom? There are a couple things that it could be like Crohn's, or ulcerative colitis. It could also be something like hemorroids, but it doesn't sound like you have been straining to have a bowel movement. I would suggest making an appointment with a GI doctor because it could be something serious as the conditions I listed above. In either case, there are treatments to help with those issues.
I am so sorry you are experiencing such symptoms. I can only imagine how scary that must be. Hang in there.