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.
In addition, if your periods were irregular before taking the pill, your periods may continue to be irregular after stopping the pill. Generally, your period should start four to six weeks after taking your last birth control pill. If you have not begun menstruating after three months, you may have a condition called post-pill amenorrhea. Birth control pills stop your body from producing hormones that trigger ovulation (egg release) and menstruation.