Was there a very foul odor coming from her rear end? That's how you can tell if it's vent gleet.
If you trim back fluff and feathers, dont trim back too much. Urates in feces will scald her skin causing redness and inflammation, probably painful too.
Is there a foul odor around her rear end? If so, it looks to be vent gleet. The blood might be from an internal broken eggshell that has pierced blood vessels or it's from other hens picking at her rear end, or both.
I agree with Chicalina, she needs to be cleaned up.