I'm sorry that your question wasn't answered in a timely manner. We don't have many avian vets on this site. Please let me know if you still need help.
It's important to note that once a budgie acts ill it's already quite ill and in need of the attention of an avian-oriented vet. This is a protective mechanism because sick birds are attacked by other birds in the wild and within the flock. Booboo's symptoms of diarrhea and regurgitation/vomiting(?) are important symptoms but they're not pathognomonic (specifically indicative) of any one particular disorder.
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 Booboo's 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 she can be attended to, please heat up her environment to 85F 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. Remove her perches and put her food and water on the bottom of the cage along with her. Add a water soluble avian vitamin such as Oasis brand 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 problem in our pet birds. 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.
I understand how difficult it can be to transition budgies over to a more appropriate diet. Here's an example of how that might be done: http://www.harrisonsbirdfoods.com/using-our-foods/small-bird-conversion/
Please respond with further questions or concerns if you wish.