Bartholin's cyst: This condition is characterized by a tender, flesh-colored lump on the labia majora, often appearing during menstruation due to increased fluid production and potential blockage of the Bartholin's gland duct. The timing and description match closely with the symptoms of a Bartholin's cyst.

Folliculitis: An inflammation of the hair follicles, which can cause tender, flesh-colored lumps and is more common in areas with hair, such as the labia majora. Menstruation can sometimes exacerbate or bring attention to existing conditions due to increased sensitivity.
Ingrown hair (pseudofolliculitis): Similar to folliculitis, ingrown hairs can cause small, tender lumps, especially in areas where hair removal is practiced. The appearance during menstruation might be coincidental but could also be related to changes in skin sensitivity.
Vulvar abscess: Although less common, an abscess in the vulvar region could present as a tender lump. The onset during menstruation could be related to increased bacterial growth or blockage of glands due to hormonal changes.

Malignancy (e.g., vulvar cancer): Although rare, any new lump, especially one that persists or grows, should be evaluated for the possibility of cancer. Early detection is crucial for effective treatment.
Sexually Transmitted Infections (STIs): Certain STIs, like genital herpes or syphilis, can cause lumps or ulcers in the genital area. These conditions require prompt diagnosis and treatment to prevent complications and transmission.
Necrotizing fasciitis: A severe, life-threatening infection that can cause pain, swelling, and tenderness in the affected area. Early recognition and treatment are critical to prevent severe outcomes.

Vulvar hidradenoma: A rare, benign tumor of the sweat glands that can present as a lump on the vulva. These are typically not tender unless they become infected.
Vulvar lymphangioma: A rare condition involving the lymphatic vessels, which can cause lumps or swelling in the vulvar area. These are usually congenital but can become apparent at any age.
Eccrine gland tumors: Rare tumors originating from the eccrine sweat glands, which can present as lumps on the vulva. They are usually benign but require evaluation to rule out malignancy.