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Any form of irregularity in periods can be due to hormonal
imbalance;and even in case of a cyst
.The reason for starting primolut-N is to regulate the periods but as according to the dose if the dose of hormones given by the pills if they are less as against the body requirement or for any reason if the blood level of the hormones is lowered than required then there is bleeding and this bleeding before time is called breakthrough bleeding
.I hope this answer helps you.