Messed up cycle since birth?

Chelsea • Married to an amazing man and Mama to a beautiful little girl. Ttc baby number two!
My cycle used to be 28-30 days and ever since I had my first period after having my daughter ten months ago they have been every 40-42 days. I told my doctor and she didn't seem concerned. Has this happened to anyone else? My daughter was born beginning of Sept and I got my first period end of December.