As I'm sure you know, the FIT test only tests for blood in the poo. Piles are one thing that can cause this and given that you know you have them, there is a good chance they are the cause. There are also loads of other possible causes - polyps, ulcerative colitis, Crohn's, certain infections... At your age, cancer is pretty unlikely. I am 41 and last summer, I was referred for the camera test for rectal bleeding and some other symptoms. The consultant said that at my age, I would be "the unluckiest person in the world," if it turned out to be cancer, that the most likely explanation was piles, with ulcerative colitis a lesser possibility. I will add that I did not have a positive FIT test, but I am older than you (bowel cancer under 30 is, I believe, incredibly rare) and the GP's examination had failed to find any indication of piles, whereas you know you have piles and constipation, which I believe can also lead to bleeding.