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.
How long does it take birth control pill to start working? My girlfriend just started taking birth control pills, and a doctor at Planned Parenthood said she only had to wait 5 days, but I've been researching it and have found either at least 7 days or as long as a month. She started taking it about a week and a half before her period.