Based upon on a bad experience of mine and anecdotal evidence from others, she is internally bleeding. If your "home tank" has had a partial (25% more or less) water change in the last week, ease her back into it. If not please make that partial water change and then, if she is in that stressful and probably toxic breeder box, get her back into the tank.

She undoubtedly will do better back in your main tank. Give her and the aquarium what TLC you can. Whether she will die or survive, whether she will never have fry again or whether she will recover and give birth to fry again, only time will tell.

Only use that box for something like temporarily holding fry you are netting out of a tank. Or toss it in the recycle barrel. Please never put an adult livebearer in one of those things again.