In the postpartum period, if you are on breast feeding exclusively, there will be lactational amenorrhea
and it gives more than 98% protection against unwanted pregnancy
. Though mild spotting or bleeding is quite common during this period due to hormonal
Second thing, irregular taking of the pill can cause the same finding and is less effective in preventing pregnancy.
Here, I suggest you to undergo one blood test
of beta-HCG to rule out product of conception if any. Following test report, you should consult with gynecologist for next line of management.