What Causes Late Period After Delivery?

I have six month baby, my period start after forty days of bleedidng but that was not lite actual one they are like brown discharge only but not in much quantity, two three months they happen same on same date, but last month instead of 12 they occur on 16 with realtively better and bleeding also there, but this month they didnt occur yet? so what can be cause ? i am very much worry?