Of course, this could be any of the many chicken diseases that are possible, but I'd say the most likely is coccidiosis, given the time line.
Chickens imported from elsewhere have not been conditioned to be resistant to the local coccidia, and since she's been on your local soil, and the incubation period is 5 to 7 days, I would guess there's a good chance this is coccidiosis.
Get some Corid, the liquid kind. It treats coccidia in all animals. Give the sick chicken a drench dose of undiluted Corid immediately, directly into her beak once a day for three days at .1ml per pound of body weight.
In addition, mix Corid at two teaspoons to one gallon of water for all to drink for the next five days, mixing fresh each day. All must have this water, including the sick one. Wait a week after the fifth day, and then start them on a new five day round to catch any eggs that have hatched. This blocks thiamine (B-1) so do not give vitamins with thiamine during treatment.
Coccidia are parasites found in all soil all over the world, hundreds of strains that target specific animals. There are nine strains alone that target just chickens, and they vary from place to place. Normally, animals raised on a local soil become resistant, but animals moved to a new locale won't be resistant to those coccidia. Rain and wetness encourages population explosions, so it's even more likely this is what's making your chicken sick, and others may be getting sick at this moment.
