Losing feathers and Dark Brown/Black underbelly

by LSM
(Hawaii)

I have a pair of finches. One male and one female. The female seems to losing feathers and had been turning dark brown/black in the underbelly and anal area. Shes still active and eating and playing on the swings, but not as much. Her poop seems to be watery and always get stuck. I separated them just incase the male was pecking her feathers, but now shes not as active. Please help. I am nervous and worried for the female.