Several infectious skin diseases also cause a rash. These can be caused by a number of infections with bacteria or viruses, and infestations of mites within the skin. Some infectious skin diseases, such as scabies, are caused by mites that bore into the skin surface causing irritation and a localised rash.
Now, as for your question about when to start taking oral contraceptives, there's really no hard-and-fast rule about when to start taking birth control pills. Starting in the first few days of your cycle is the surest way to prevent inadvertently taking pills while you are pregnant.
You can start progestin-only pills at any time. You'll be protected from pregnancy after 48 hours (2 days). So use another method of birth control (like condoms) if you have vaginal sex during the first 48 hours. Your nurse or doctor can help you figure out the best time to start your birth control pills, and when they'll start working for you.