There is blood in my poop? help?

I just went to the toilet and took a poo. There was some blood in my poop and this has never happened to me before. I have not had , ever. I am taking iron tablet but have been for a long time now and they havent caused any problems, (except i get diarreah sometimes and sometimes im a bit constipated but i dont force it out, i just drink a lot of water and if i wait a day it comes out really easily and turns into diarreah) and if it helps i have bi-lateral duplex systems. (its a disease in the kidneys, on is larger then the other and it has extra tubes connecting to the bladder etc.) Also i have been having a very sore stomach for the past few days, mostly just the pain when you need to poop a bit, but right now i have a pain that feels like a hunger pain even though i ate my dinner not very long ago (and i eat a lot, and i know it isnt a pain from being full) . My mum went to the shops so i cant really ask her right now, help please??