Missed period after birth

Mi

Hey girls my little dumpling is 5 months old and I got my first period last month. Today my period is 4 days late and I’m not sure why. I also got a negative pregnancy test about a week ago. The pregnancy test said it can detect pregnancy up to 5 days before missed period. I don’t breastfeed anymore and I also take birth control. Anybody have any idea what going on or has this happened to you?