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.
To take the Pill, follow the instructions on the package. Your health care provider will explain how to use your pill pack. You will be told to start taking the birth control pill on a Sunday, on the first day of your menstrual period, or the day you are seen by your health care provider. You should take one pill each day, at the same time of day until you finish the pack.