I don't think you are being irrational, but I do think bowel cancer is unlikely. You have a pretty fair idea where the bleeding is coming from and while yes, there are cases of misdiagnosis, those are firstly, using cases where the fissure was not confirmed; either the patient or the doctor just assume "probably a fissure" and didn't investigate further. 

I think it is good that you are going to get it checked out, just on the off-chance there is something going on, but the odds are that it's just the fissure and maybe stress. Stress can cause a lot of bowel changes and stomach problems and you are bound to be under stress given your dad's diagnosis and your own anxiety about developing the same thing. This could also make you more aware of little things. Bowel and stomach problems are common and it's quite possible some of those things are ordinary, time-of-the-month or something-you-ate changes that are simply taking on greater significance due to the whole situation.

I do think it would be a pretty big coincidence if you developed the very condition you were worried about before having symptoms and after developing an anal fissure that caused one of the symptoms of that condition.

I will add that I have read that little spots of blood on the tissue are usually either hemorrhoids or a fissure, so given that you have one of those, it would be pretty unlikely anything else was causing the bleeding.

And that I had similar symptoms last summer - except that they could not find a fissure or anything else to explain the bleeding, which was when I started getting worried. Bleeding (red, on tissue but sometimes more than spots), weight loss, occasional loose stools (maybe once a week), trapped wind and a weird pain under my ribs (which I actually think was just due to my phone poking me there when it was in my pocket!!). I was referred for a colonoscopy and when I asked the specialist how worried I should be, he said, "zero, not at all," that he would be concerned if my mother came in with the same symptoms, but that at 40, I'd be "the unluckiest person in the world" if it turned out to be cancer and that it was probably hemorrhoids but he did want to do a colonoscopy on the off-chance of colitis.

Bowel cancer in the family does raise the risk a little so you should probably mention your dad's diagnosis when you see the doctor, but there are many things more likely than cancer here.

I don't think you are a hypocondriac. I think you have had a really upsetting situation with your dad and it's only natural that you would be concerned about similar symptoms, given the result in his case. Anybody would be. But that doesn't mean it's likely for you.