I understand completely how you feel. From what you've said, it sounds very, very likely that this is a fissure or piles.

If it is any consolation, a year ago, I had rectal bleeding on a regular basis for months, treatment for piles made no difference to it and when I saw a GP, they saw no sign of piles or a fissure. I had also lost about a stone in the previous year and had some stomach issues, loose stools and trapped wind (probably at least partly due to the stress of the whole situation).

I was referred for a colonoscopy and the consultant said that given my age (I was 40 at the time), bowel cancer was the least likely of all the options and that it was probably piles but he would do a colonoscopy because there was a small chance of IBD. It turned out to be piles.

In your case, given that you have reason to believe you have a fissure and that it has only been going on for a couple of weeks and you are pregnant, when I believe piles are more common, I would say the odds of cancer are even lower than in my case and the consultant said the odds for me were less than 1% and I would be "the unluckiest person in the world" if that turned out to be the explanation.

Like you, I was freaked. I knew the odds were against cancer, but even a small chance is scary.

You are doing the right thing by contacting your GP and hopefully, he or she will be able to put your mind at rest.

Thank you so much for replying! I'm sorry to hear the experience you had with rectal bleeding but so happy to hear everything was ok! It's just so scary seeing blood come from a place it isn't supposed to. I know  rationally it's probably piles or fissures but stupid anxiety brain won't allow me to believe that to be the cause! 
Your story has given me some hope! Thank you  again for sharing!