Based on your symptoms and the fact that you are young, it sounds that you may have an anal fissure
together with hemorrhoids which can bleed significantly, particularly when you go to the bathroom.
It is difficult to tell you the exact reason without having a proper examination, which should include AT LEAST an anoscopy, but preferrably a flexible sigmoidoscopy
or a colonoscopy
If you are known to have hemorrhoids, you can try in the meantime "Sitz baths" (sit in warm water, in a tub if possible, for 15 minutes twice a day; you can add Epson salts to the water while you are sitting). Don't apply any cream or suppository until you see a physician as this can worsen your possible anal fissure.