It seems like you are suffering from a retro caecal appendicitis which is difficult to be diagnosed by a USG and hence the delay in the operation. The other more plausible explanation is that the infected appendix might have already ruptured causing an appendicle abscess. Appendicle abscess are difficult to visualize in its earlier stages although it can be identified within a week of rupture.
Appendicle abscess is a collection of pus in the appendix that occurs when acute appendicitis is not managed appropriately
I would recommend the following-
1)Consult a radiologist and get an ultrasonography of the abdomen. I t will not only help in the diagnosis of the appendicle abscess but also get information on the size, shape, location of the abscess. These information help in deciding the further course of action.
2)The patient has to be started on long term antibiotic like Tab cefepim and Tab. Clindamycin. If she cannot take orally, intravenous treatment has to be started.
3)After intravenous (IV) antibiotic therapy, an interval appendectomy can be performed 4-6 weeks later.
4)Patients with a larger well-defined abscess: After per cutaneous drainage with IV antibiotics are performed, the patient can be discharged with the catheter in place. Interval appendectomy can be performed after the fistula is closed.
5)Adequate parenteral or IV nutrition needs to be given to prevent dehydration.