Hi welcome to health care magic forum. You have taken plan b
and immediately you had foul smelling brown discharge and after 3 days period started. It appears that it is only a coincidence if it is due to plan b or some thing else it will be only temporary, and smell is continuing during the period also means that there is some infection and that is is continuing. Your doctor told long back that you are healthy, but the infection has started now.So yougot the symptoms now. if you don't get treated same thing continues in the next period also, and may get brown discharge through out the period.
Of course it is a common infection but it should be treated, so i advise you to consult a gynaecolagist for diagnosis and treatment. After that you will be as healthy as before.
Besides i advise you some diet recommendations as avoid taking pulses, potato other tubers, and fish.
Take more of green leafy vegetables,sprouts, milk, egg, meat, fruits, juices, and dry fruits to improve blood, general health, and resistance to such infections. Wishing for a complete recovery and thanks for calling.