From what I have read, narrow stools on their own are very, very rarely cancer. And it would be very unusual to get a normal FIT reading with bowel cancer - I've read that the odds of bowel cancer with a normal FIT are like 1% or something. You would also be young for bowel cancer, though of course, it's not impossible to get it in your 20s, but to get it in your 20s and have a normal FIT would be very unusual and especially if your only symptom is narrow stools. As far as I can see, there seems to be debate as to whether they should even be considered a cancer symptom.