She may have been ill prior to you getting her, and it's just catching up. For now, just keep her separated and warm to see if she perks up. Her suddenness of it does concern me, as that sort of behavior change, along with the lethargy could be an indicator something is just not right with her. But her not fading in coloration is odd if she were sick.. silly fish, if only they could talk and tell us where it hurts.
But without other symptoms or signs, I would not feel comfortable recommending any medication as the wrong treatment could be more detrimental then what is going on with her now.
She could of also gotten put in her place by another and she is just hanging low for a bit.. lets hope it's something simple as that.
Poor little girl.. I hope she pulls through, females have a soft spot in my heart hehe.
If need be, and I recommend, getting a 5th female and placing her in the tank. It's fine to have 5 in a 10 gallon, and if the sick girlie needs to stay out for a while, the other female would help keep the 3 left in the tank from harming each other. 3 females together is a dangerous number.