You either accidentaly pressed restart, or yours was an adult and it had a baby or, there was a glitch (which happens when you have your Tamagotchi for a very long time.) And if it was your first gen. then you wouldn't have had a Mimitchi anyways.
i had the same kind of problem it was so weird. i had an oniontchi and it went to a baby. it even did adult poop! note: i have a chohimetchi. :furawatchi: