Period lasted 4 day's never happened before

amy • 2 kids,1 angel baby,#3 on the way married 11 years
OK I never had a period last 4 days it always last 7 days I had a baby March 22/16 so when my postpartum bleeding stopped at week 3 I got my period week 4 on April 21/16 it lasted 9 days. I don't understand why my period just stopped.