That's good. The worming and coccidia medicines that enola mentioned may be called something else in Canada. Is there a local byc person who knows what is available in Ontario? How long has it been that she is losing weight? How long ago did you notice the poopy/dirty rear end?
Have you felt her crop? Is it hard and full of food? She may have an impacted crop.
When you let her outside, it is more likely she is expending energy on the appearance of being healthy. She should be kept warm and isolated so she can save her strength.