Bh, I'm a hypochondriac too. Just now for instance I've had a flu virus which has gone on and on and left me with a continual cough. Because I've smoked on and off all my life though given up since pregnant (over 3 years ago now, dd's 2.5) I started suspecting lung cancer. I know it sounds daft and dp actually laughed when I told him. But I've spent a couple of weeks mentally visiting my own funeral and imagining my horrible, slow, pointless death and driving myself crazy.
If it's any consolation re. your worries, and this is going to be straightforwardly graphic, if you pass very bright red blood and especially if it's on the toilet paper rather than in the toilet, the likelihood is that it's very close to the anus. And if your GP has actually felt that you have piles and this is a long term thing with no weight loss, low left-hand sided pain, no change in appetite etc. I think you can put your mind at rest. As best you can of course!
My Ex dh's mum and grandmother both died of cancer of the colon a year apart from each other. In both cases it was very quick, from the moment they started getting diarrhea and constipation constantly alternating with pain and bleeding, and were then diagnosed they had died within 3 months.
I'm intending this to be reassuring rather than scary.
Incidentally I think I've become SO much more afraid of being seriously unwell since becoming a mother. It's the thought that you're so needed and so responsible and that without you everything will fall apart.