All grown up??

Starting last Tuesday Calli has begun to call me 'Mom' instead of 'Mommy.' I really can't figure out where she picked up on it - the girl is only two! I decided I don't really like being called Mom instead of Mommy. The other day I asked, "Calli, will you call me Mommy?" With all seriousness she abruptly responded "No." What is a mommy to do?!