It's likely that stress has caused her to stop laying, but just in case, can you feel an egg around her vent? The lack of poop may mean that she's egg bound.
If she is egg bound, you want to get some calcium citrate into her as soon as you can and give her a warm (quail body temp or a couple degrees warmer) epsom salt bath. If she is egg bound, it will kill her quickly, so it's important that she pass the egg as soon as possible.
Is she fluffed up at all? Is her activity level normal other than walking backwards?
Is there a chance that the other quail is a male? This sort of injury is usually male on male or over breeding, but that doesn't mean that hens won't peck and cause injury, it's just less likely.
Have you been providing calcium on the side?
This isn't related to her symptoms, but at 4 months, she should be transitioning over to an adult feed in the 20-24% protein range.