Endometritis or Pelvic Cellulitis: This is the most likely diagnosis given the patient's presentation 9 days post-hysterectomy with moderate vaginal discharge that has a brown tinge and a foul odor. The symptoms suggest an infection of the upper genital tract, which is a common complication following hysterectomy.

Vaginal Vault Infection or Wound Infection: Infections at the site of the vaginal vault or surgical wound can present with similar symptoms, including discharge and odor.
Urinary Tract Infection (UTI): Although UTIs typically present with dysuria, frequency, and urgency, in the postoperative period, the symptoms can be masked or atypical, and a foul odor can be present if the infection involves the upper urinary tract.

Necrotizing Fasciitis: Although rare, this is a life-threatening condition that requires immediate recognition and treatment. It can present with signs of infection, including foul odor, but also with severe pain out of proportion to the physical findings and signs of skin necrosis.
Septic Pelvic Thrombophlebitis: This condition involves infection and clotting of the pelvic veins and can present with persistent fever and signs of infection not responding to standard antibiotic therapy. It's a rare but serious complication that can occur postpartum or postoperatively.

Retained Foreign Body (e.g., Surgical Sponge): A rare but possible cause of infection and foul odor, especially if a surgical sponge or other material was inadvertently left in the pelvic cavity during surgery.
Fistula Formation: Early signs of a fistula (e.g., vesicovaginal or rectovaginal) could include abnormal discharge, but this would typically be more consistent in nature (e.g., urine or stool) rather than purulent discharge with a foul odor. However, in the early stages, the presentation might be atypical.