Bartholin's cyst or abscess: This is the most common cause of a tender nodule in the labia, resulting from obstruction of the Bartholin's gland duct, leading to cyst formation or infection.

Folliculitis: An inflammation of the hair follicles, which can cause tender nodules in the labial area.
Ingrown hair (pseudofolliculitis): Can cause a tender nodule, especially if it becomes infected.
Vulvar cysts: Such as epidermal inclusion cysts or sebaceous cysts, which can become inflamed and tender.
Herpes simplex virus (HSV) infection: Can cause painful vesicles or ulcers in the genital area, which might initially present as tender nodules.

Malignancy (e.g., vulvar cancer): Although rare, any persistent or growing nodule should be evaluated for malignancy to ensure early detection and treatment.
Sexually transmitted infections (STIs) like syphilis or chancroid: These can cause ulcers or nodules in the genital area and have significant health implications if not treated promptly.
Necrotizing fasciitis: A severe, life-threatening infection that can start with a small, seemingly insignificant lesion but rapidly progresses.

Vulvar hidradenitis suppurativa: A chronic skin condition that can cause painful nodules and abscesses in the apocrine-gland-bearing areas of the body, including the vulva.
Erythema nodosum: An inflammatory condition that can cause painful nodules, typically on the lower legs but occasionally in other areas, including the vulva.
Foreign body reaction: If a foreign object is inserted into the vagina or surrounding areas, it can cause a localized reaction, including a tender nodule.