Hi,
Even though celery is on some bunny veggie lists it should be in limited quantity..one cup is too much, because its a lighter vegitable it is not that good to give in larger quantities as in this case, it will lead to poopy but which it has.
The key to veggies is to offer 3 different daily so they get different nutrition. Too much of one thing is not good can cause them to get sick and they dont get nutrition that they need from other greens. Dark leafy greens are important to give as well. Some greens that are good to try are cilantro, dill, basil, mint, romaine red or green leaf lettuce, endive, swiss chard red or green, parsely, kale, carrot tops. The last three mentioned should be given in moderation too much given over long period of time can lead to kidney and sludge problems because contain too much calcium.
I think you should take celery out of her diet does not have much nutritional benefit and it is causing her poopy but. You can try replacing it with some of the greens stated above, slowly introduce them.