From what I understand the burning is stomach acids that have not had time to neutralize (because of the ES purge).
Regarding blood -- as a kid I'd have bloody stools. Of course, I never told anyone. I later read about it and it's all along the lines of burst capilaries due to straining (people even burst capilaries in their faces and eyes from straining.)
It's the occulted blood that worrisome -- the blood you wouldn't even see, b/c it's turned dark inside your intestines. A little red blood (it looks like a lot more in the toilet) isn't anything to worry about. I stand as proof!
Of course, you could get it checked out if it does worry you.