I'm sorry to hear about this with Doris.
It's important to note that once a cockatiel acts ill it's already quite ill and in need of the attention of an avian-oriented vet (please see here: theparrotsocietyuk.org/veterinaryadvice/avian-vets). This is a protective mechanism because sick birds are attacked by other birds in the wild. Doris' clinical signs of bottom dwelling (weakness) and anorexia are important signs but they're not pathognomonic (specifically indicative) of any one particular disorder. They represent “sick bird syndrome” - acute onset illness with an array of highly nonspecific clinical signs.
An avian-oriented vet might first treat symptomatically and supportively by providing supplemental fluids and electrolytes by needle and tube feeding a "recovery" food. Blood tests and cultures of Doris' choana - the slit between her oral cavity and nose - and cloaca (vent) may be taken. Whole body X-rays can be helpful as well.
Until Doris can be attended to, please continue to heat up her environment to 29.4C by means of a 100W bulb shined into her partially covered cage (not at night when she needs to rest) or by taping a heating pad set on its lowest setting to the sides of her cage. If she appears weakened, remove her perches and put her food and water on the bottom of the cage along with her. Continue to add a water-soluble avian vitamin such as Oasis brand Vita-Drops to her water at half of the recommended dose so as not to make her water distasteful. Add a calcium supplement such as Calcivet or Calciboost to her water. These supplements are available in pet/feed stores. Avoid over the counter antibiotics designed to be placed in her water. They won't be effective if only because an ill bird won't drink enough to medicate itself properly.
Nutritional imbalances are a common cause of illness in our pet birds. What has Doris' diet consisted of, please? Seeds should comprise 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 - yes these low lactose foods are safe, 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.
For tricks in transitioning her to a formulated diet please see here: https://www.harrisonsbirdfoods.com/using-our-foods/diet-conversion/ and click on “Feeding Harrison's”.
Please respond with further questions or concerns if you wish.