It is probably nothing, one time reddish tinted string in stool may be anything from your diet.
But most of the serious illness associated with colon
And people always dont have the habit of looking on there stools so they miss much.
You should avoid taking fast foods and try add plenty of fibers to your diet. Have lots of vegetables and adequate water.
It is always better to consult a surgeon and get examined for any changes in rectum
I am not trying to scare you here but it is better be early.