There are two main considerations when "pasty butt" is seen. The first is a diarrhea whose consistency predisposes to droppings attaching to and accumulating on the cloaca (vent). The second is an obstruction of the distal colon causing retention of droppings and then abnormal expulsion of them. This is most often caused by egg binding which needs to be an important consideration in a cockatiel (?) who has been such a prolific egg layer and now appears swollen in her lower belly perhaps caused by a retained egg. If her diet is deficient in either vitamin D3 or calcium, her oviduct will have weakened and lost its ability to pass eggs efficient. What has her diet consisted of, please? Seeds should compose less than 20% of her diet. A diet of mainly seed and nuts has excessive fat, carbohydrates, and phosphorus; marginal protein; adequate vitamin E, and are deficient in amino acids, calcium, available phosphorus, sodium, manganese, zinc, iron, vitamins A, D3 (necessary for efficient absorption of calcium), K, and B12, riboflavin, pantothenic acid, choline, and available niacin. Ideally, a balanced pelleted diet such as can be found here: www.harrisonsbirdfoods.com or here: www.lafeber.com/pet-birds should be fed as well as hard boiled egg yolk, pancakes and cornbread, the tops of fresh greens, dairy products such as yogurt and cheese, fresh fruits such as apples, pears, melon, kiwi, and berries, vegetables such as broccoli, carrots, beets, asparagus, cabbage, sweet potato, and squash, and even tiny pieces of meat.
Immediate therapy for egg bound birds involves heating their environment to 85F by means of a 100W bulb shined into a partially covered cage (not at night when she needs to sleep) or by taping a heating pad set on its lowest setting and taped to the sides of the cage. Additional calcium should be provided in the form of a supplement placed in the water such as Calciboost or Calcivet available where bird supplies are sold. If an egg isn't delivered within 24 hours of doing the above, an avian vet (please see here: www.aav.org) will be necessary. Please see here for more information in that regard: https://vcahospitals.com/know-your-pet/egg-binding-in-birds
Please respond with further questions or concerns if you wish.