I am assuming that you're getting an illness and misadventure form because you didn't perform to your potential due to an illness, and not because you didn't show up to your exam, right?
I don't think you need a doctor's certificate that was issued on that day (that isn't what is required at my school). However, it would most worthwhile to get it today, or tomorrow, to ensure that that you are ill now.
I honestly don't think it'll be a problem if the doctor's certificate is issued the day after