A few hundred years ago a novelist was writing about someone investigating a noise outdoors. When that person came back they reported that the noise was a right as rain. This indicated that any noise created by rain would be normal enough, and so were the noises outside. Just normal.
This phrase started to get used more often as the years went by until it eventually came to mean feeling better after a period of illness.
So that is where I am at, hoping that in the next day or two I will be right as rain.