Ill do water changes more regularly.
While all the input so far is excellent, and I 2nd on the feeder fish, they are almost guaranteed to have parasites at some point or another. She could also be "fish-pressed".....never saw a male do it, but I have had females who wanted to breed exhibit strange behavior after awhile which I interpreted as depression from not having a mate. While there's no scientific evidence to back this up and it may sound ridiculous, I have no doubt it's possible considering I never actually FOUND anything wrong with her. Just something to consider once she is back to normal and healthy again.
Personally, I would do 50-70% water changes weekly, and add/re-add 1 Tablespoon of salt per 5 Gallons of tank water. I keep salt in my tanks 24/7, 365 and rarely have any pathogen issues. If it is internal parasites, a stronger more direct approach such as the aforementioned medications will be needed.
For now, I would remove feeder fish off the menu permanently and replace them with earthworms or waxworms or butterworms, and try to work some peas, lima beans, or minced greens into her diet....some Jags will eat them, some won't. Shrimp, clams, mussels, oysters, (schucked), and other shellfish can be a safer healthier treat rather than feeder fish, but crickets are great.