Oh, I think it was from a hemorrhoid bleeding. This is because when you have some constipation and your waste gets hard and even enlarged, it will go past those delicate tissues and make them bleed, and it'll show up on toilet paper just as you described. Hemorrhoids come from straining because of constipation, the tissues swell up and will bleed and hurt when hard waste is pushed past them. The three fixes for constipation are: Enough water, enough fiber foods, and regular exercise. Beyond that, medications can constipate a person, review side effects of anything you're on, and so can daily vitamins, so you can switch brands or only take them a couple times a week, and if you recently went thru a round of antibiotics, they can constipate too, and the solution is to return the "good" bacteria the medicine kills by consuming probiotics, which is in Acidophilus milk or Activia yogurt at the grocery store for a couple weeks.
If you experienced a hard bowel movement, it's possible the blood you saw could be from that. You may want to wait a few days to see if it happens again. If you do find the problem is repeated, call your doc and ask to be seen.