Usually rashes appear during periods. If it appears before period it might be a symptom of the upcoming period. Ok, whether it appears before or after, maintaining proper hygiene can help prevent them. Take bath twice daily ,use clean and comfortable garments. Drink lots of water to keep oneself hydrated. Have juices and fruits .This may prevent the break out of rashes. Have nutrient rich diet. You can also start using creams or ointments that cures rashes 2 to 3 days before the start of the period. Several anti- microbial creams specifically for this problem are available .They will soothe the skin.