The symptoms suggest intertrigo, likely complicated by candidal (fungal) infection—a common concern in areas with friction, moisture, and warmth, especially between the breasts.
Next Steps
Keep the area dry, wear loose cotton bras, and use antifungal dusting powders. An antifungal cream with mild anti-inflammatory action may help—but exact treatment should be tailored based on skin type and severity.
Health Tips
To prevent recurrence and manage pigmentation, timely and correct therapy is crucial. For a precise diagnosis and prescription-based care, I strongly recommend a detailed online consultation with a dermatologist.