Hi Annya - welcome to the board!
first off, don't be worried about being too graphic on here - as you say after all, that's what this board is about! it soon becomes second nature lol!
As for your symptoms, well i know narrow stools can be a sign of ibs (as i have them myself) but bleeding from inside the colon is not. As you say it could be hemmies (especially if the blood is bright) But it also might be indictive of another disorder like UC or Chrones
As you say you are quite young and it would be unlikely that you have a tumour (or probably anything more 'serious' than ibs for that matter), but because your bathroom habits have changed and also because you are seeing blood in your poops you should check it out - if only to put your mind at rest.
i would recommend seeing your doc who might arrange some tests for you if he/she feels you need them...