Positive Fit test :( Blood found in stool but wasn't visible at the time. Why is that?

Got the dreaded result back from the Fit test I taken for Bowel cancer detection that showed I had blood in the stool, though there was no visible blood at the time when I sent the sample off nor did I have any bleeding when I went to the toilet or any symptoms at all that may indicate bowel cancer - anyone been through similar experience ? So I'm terrified, confused now, of course I will do further tests at the hospital etc now, my heads just all over the place !!! Not great as my dad had bowel cancer but he did have symptoms before that hence how I am feeling now .

And why is it some blood invisible in stool ? Unfortunately I didn't ask for the results in detail as first it was the shock of what I was told 'positive' for blood in stool and secondly I just assumed the results would be either positive or negative.

Any advise or anyone been in similar situation ? Of course I'm not asking for answers as no one here can give me that or tell me what it is or isn't !!