Bacterial Vaginosis (BV): Although the discharge is described as green and rotten smelling, which is less typical for BV (usually fishy smelling), the onset with soap use and absence of other symptoms like itching or burning could still point towards a disruption in the vaginal flora. BV is a common condition that can present with a variety of symptoms and is often associated with an imbalance in the vaginal microbiome.

Trichomoniasis: This sexually transmitted infection can cause a green, frothy, and foul-smelling vaginal discharge. The fact that the patient is monogamous does not entirely rule out this diagnosis, as the infection could have been present before the current relationship.
Vaginal Candidiasis (Yeast Infection): While typically associated with itching and burning, some cases can present with a foul-smelling discharge, especially if there's a secondary bacterial infection. The use of soap could potentially disrupt the vaginal pH, contributing to the development of candidiasis.
Atrophic Vaginitis: This condition, often seen in postmenopausal women, can cause a thin, dry, and sometimes foul-smelling discharge due to the decrease in estrogen levels. However, it's less likely in premenopausal women unless they have other risk factors for estrogen deficiency.

Cervical or Endometrial Cancer: Although rare and unlikely, any abnormal vaginal discharge, especially if it's persistent and accompanied by other symptoms like abnormal vaginal bleeding, should prompt an investigation for malignancies. The absence of other symptoms does not rule out these conditions entirely.
Pelvic Inflammatory Disease (PID): This condition can cause a variety of symptoms, including abnormal discharge, and is a significant concern due to potential long-term consequences like infertility. The absence of pelvic pain does not rule out PID, as some cases can be asymptomatic or present with minimal symptoms.

Foreign Body: A retained foreign body in the vagina, such as a forgotten tampon, can cause a foul-smelling discharge. This is more likely if the patient uses tampons and has a history of forgetting to remove them.
Vaginal Fistula: An abnormal connection between the vagina and another organ (like the intestine or bladder) can cause persistent, foul-smelling discharge. This condition is rare and usually associated with a history of surgery, trauma, or certain diseases.