How have subsequent poops been? If it was just a weird one-time thing, I wouldn't worry about
it, probably from something you ate, but if you don't eat exactly the same things ever day but the poop still has red stringy stuff in it, I would definitely be wondering what I could be. (I've noticed a few veggie type things that don't get fully digested, like corn, tomato skins, grapefruit, blueberry skins... that come out looking weird in my poo...)