Was the poop stringy or chunky? Usually if it's chunky, it's not parasites.
Start her on some epsom salt at 1 tsp/gal to get her to start pooping. This will help you see if it's an issue or just a one poop thing.
With females, they often either drop eggs and eat them, causing white poop, or absorb them internally, also causing white poop.
Get an parasite medication to have on hand just in case, but watch her for another day before treatment.
You can only prevent parasites with good conditions as they are always in the aquarium or fish. They can only infect if the immune system is weakened, so good conditions usually prevent it.