Looking at the weather for your area, it the temperatures are mild and pleasant but rainy lately. If I had to take a guess, I'd think cocci overload. Rain tends to bring the eggs and parasites to the surface, where it's easily digested by your flock. Cocci is/are a one-celled parasite found in one sub-species or another in every square inch of soil on the planet and preys on young, old or birds that are 'at risk' due to hidden illness or disease. Some varieties of cocci cause bloody stool, others do not. The treatment is amprolium (generic name), use the 'outbreak' dosage listed on the packaging and offer it in ALL water sources that your flock has access to. The medication works by imitating the B-vitamins that the parasite uses as food, so do not offer B-vitamin supplementation while you're treating your flock but DO offer it after the full course of treatment. There is no other down-side to treatment, and cannot harm your flock or their eggs in any way, shape, or form.
Cannot say with certainty that this is the problem, but with your weather conditions, this is where I'd start because even if she has a more serious issue, the parasite is opportunistic and could place her in an 'at risk' category. Treatment for coccidiosis first, will enable you to determine if there is more going on with her and give any further treatments for other illness (if found) a head-start by reducing her load of cocci.
Other internal parasites may also be at play, some visible to the naked eye, some not. A vet will likely be glad to do a fecal float test for a variety of worms.
Keep us posted, please.
