This sounds similar to me.

I am awaiting colonoscopy.

For a few years now, like 8 or more i have periods once a year where I wake with extreme cramps, like screaming in pain cramps, i have normal stool, then diarrhea and then its just blood, the cramps ease when I pass the blood until they come in again which is quite fequent. It lasts beyween 1-3 days so its a lot of blood. Sometimes I vomit aswell and then I go back to normal.until the nest time.

Been hospitalised 2 or 3 times with it but because it stops they have always said its a bug and sent me home , never taking into account the history.

Ive not spoke to anyone else yet who experiences blood without stool so to read your post was a suprise.

Im sorry I dont have answers for you but just wanted you to know you arent alone.

Gem 

Sounds exactly like me, that's weird that you have periods once a year mind, I hope that is getting sorted also. I don't have them so I'm lucky being a bloke, but everything else I have, the stomach cramps and the number 2s are all the same. It is really painfull and worrying. Can't believe the sigmoidoscopy show everything is fine, hopefully they rush this colonoscopy for the both of us, I recommend sedation as the sigmoidoscopy is Just under half way, being a bloke thought I was hard hehehe it bloody hurt bad so take sedation, I am next time. I really think they are pollyps which are like skin tags which bleed when passing, it's like a build up of blood that's why the first one is sort of normal followed by blood. But these polyps do turn into cancer or can turn so it's best to sort it asap. That's all the research I came up with, as it's not piles or anything else. Don't think it's chrones disease. Just eat loads I recon then you got some to loose of it's serious. Hope everything work out for you and me. I will message on here with the end result.