Hormonal changes due to Nexplanon: Nexplanon, a progestin-only contraceptive implant, can cause changes in menstrual bleeding patterns, including irregular spotting or brown discharge, especially during the initial months after insertion. This is a common side effect and often resolves on its own.

Infection (e.g., bacterial vaginosis, yeast infection): Although less common, infections can cause changes in vaginal discharge. The presence of brown discharge could be a sign of an underlying infection, especially if accompanied by other symptoms like itching, odor, or burning.
Cervical polyps or cervical ectropion: These conditions can cause irregular vaginal bleeding or discharge, which may appear brown due to the presence of old blood.
Irritation or allergic reaction: Local irritation or an allergic reaction to the Nexplanon implant or other products (like soaps or douches) can lead to changes in vaginal discharge.

Cervical cancer or precancerous lesions: Although rare, it's crucial to rule out cervical cancer or precancerous lesions, especially if the discharge is persistent, accompanied by other symptoms like pelvic pain, or if the patient is overdue for a cervical screening.
Endometrial cancer: Unusual vaginal bleeding or discharge can be a symptom of endometrial cancer, particularly in older women or those with risk factors like obesity or unopposed estrogen exposure.
Pelvic inflammatory disease (PID): PID can cause abnormal vaginal discharge, and while it's less likely to present with brown discharge alone, it's a critical diagnosis to consider due to its potential for severe complications.

Vaginal cancer: This is a rare type of cancer that can cause abnormal vaginal bleeding or discharge.
Foreign body in the vagina: A retained foreign object can cause infection, discharge, and other symptoms.
Endometriosis: While endometriosis can cause irregular bleeding, it's less commonly associated with brown vaginal discharge as a primary symptom.