Guppy with a problem. Flared gills and swimming circles?

I have been looking around but cannot find an answer to this one. Maybe you can help.

I have a tank of feeder guppies and one of my females is displaying unusual coloration and behavior.

Her color over the last 24 hours has darkened to a dark bronze color while the other fish remain silvery.

Her gills are flared wide and are bright red in color.

She jerks to the left at 3-4 second intervals and is swimming in circles as a result. She is also bumping into things when she jerks.

I have added freshwater salt to the water to try and soothe her That was 12 hours ago and it doesn't seem to have helped. The day before this behavior began the tank got its regular water change. Ammonia and all other tests check out okay so nothing in the water quality has changed. None of the other guppies exhibit this behavior, only her. I see nothing clinging to or growing on her body. She hs no problems with motion and is all over the tank, but remaining out of the strong current.

She picked at the food last feeding and didn't seem to have much of an appetite. The only thing new to the aquarium recently was a piece of cucumber which all the fish chewed on. I do not recall if the jerking started before or after I added the cucumber. The timing was close. Over the last 24 hours she has only gotten worse. The cucumber has not been in the tank for the last 6 hours.

Links, advice, recommendations welcome.

Thank you in advance.