Pelvic Abscess or Infection: Given the patient's recent history of UTI, constipation, and the presence of a colostomy, a pelvic abscess or infection is highly plausible. The symptoms of heavy yellow vaginal discharge and right low back pain, along with the finding of a fullness and possible ballotable mass, support this diagnosis.

Diverticulitis or Complications from Diverticular Disease: Although the patient has a colostomy in place for 30 years due to complications from diverticular disease, it's possible that there are residual or new complications contributing to her current symptoms.
Ovarian Cyst or Tumor: The presence of a ballotable mass in the right adnexal/iliac fossa could suggest an ovarian cyst or tumor, which could cause the observed symptoms.
Vaginal or Cervical Infection: The heavy yellow vaginal discharge could be indicative of a vaginal or cervical infection, which might be related to or independent of the other symptoms.

Appendicitis or Appendiceal Mass: Although less common in this age group and with the presence of a colostomy, appendicitis or an appendiceal mass could present with right lower quadrant pain and should not be missed due to its potential for severe complications.
Kidney Stone or Pyelonephritis: Given the patient's recent UTI, the possibility of a kidney stone or pyelonephritis, which could cause severe back pain and potentially contribute to the vaginal discharge if there's a fistula, should be considered.
Colovesical or Colovaginal Fistula: A complication of diverticular disease or other pelvic conditions, a fistula could explain the combination of symptoms, including the heavy vaginal discharge.

Gynecologic Malignancy: Although less likely, given the patient's age and symptoms, various gynecologic malignancies (e.g., ovarian, cervical, or endometrial cancer) could present with similar symptoms and should be considered in the differential diagnosis.
Actinomycosis: A rare bacterial infection that can cause chronic pelvic pain, discharge, and masses, especially in the setting of a long-standing intrauterine device (IUD) or other foreign bodies, which might not be directly relevant here but remains a rare consideration.
Tuberculosis: Although rare in many populations, tuberculosis can affect the genital tract and cause chronic symptoms similar to those described, including discharge and pelvic masses.