My guess is that she is laying internally, and that the yellow is egg matter. Mycoplasma (MG) is one of several bacteria that can cause infection in the egg tract, E.coli is the other. This is very common in backyard flocks. Since most antibiotics are not approved for poultry, I would just make her comfortable and try to keep her eating. If you feel that she might be getting an overload of coccidiosis, you can try some Corid for that. What did you use for worming?