I would go ahead and treat her with Corid for coccidiosis. At her age that is the most common problem, and since she has diarrhea with red, is not eating, and is puffed up, she fits the symptoms. Give 1 1/2 tsp of the powder per gallon of water for 5 days. If you have had Mareks in your flock, then one of the problems of it is that it reduces the immunity to common diseases. After treating her, I would give her a few days of chicken vitamins and some plain probiotic yogurt, or some poultry nutridrench 2 ml daily. Sometimes having a few fresh droppings examined by a vet for worms and coccidia can save some time. Enteritis can also affeact chickens who have had cocci.