• Menstruation - particular just before the start of a woman's cycle.
• High humidity
• Local irritation or friction
• Cosmetic make-up - Look for non-comedogenic forms

Acne flare-ups just prior to menstruation are due in part to hormonal effects (particularly progesterone) but also changes in sebaceous activity and reduction in the size of the orifice of the pilosebaceous duct.

Hydration, in cases of high humidity, has shown to reduce the size of the opening of the sebaceus channel.

Rough or occlusive clothing, headgear straps, tight-fitting athletic equipment, and even prolonged resting of the hand to the chin or cheek can contribute to acne.

Quality make-up should state that it is "non-comedogenic" on the label.  If not, then it will probably contribute to acne formation.