Question about depo and how long it takes to get back to normal?

I am curios to know if I am supposed to begin counting the months beginning with the day of the last injection or from when it ran out. I was to get my shot last Wednesday but didn't we are trying to conceive and like everyone else we were not told of the side effects only that I could use it while nursing. So I guess I begin the countdown of months but am not sure which date to use. Can anyone help?