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The contraceptive efficacy of the Norethisterone is quite less and if you are taking it irregularly then also it may cause a breakthrough bleeding.
So the important thing is to get a pregnancy test as this is the most common cause to rule out. If this is ruled out then irregular intake of hormones is the second cause.
I suggest you get a gynecological ultrasound and a serum beta hCG (human chorionic gonadotropin) done. AUB (abnormal uterine bleeding), pregnancy, and hormonal imbalance are the differential diagnoses.
The treatment plan is to stop taking the pills at present. Let us know the reports of the investigation.
Preventive measures include the use of a combination of oral pills.
Follow up with the reports.
For more information consult an obstetrician and gynaecologist online -->https://www.icliniq.com/ask-a-doctor-online/obstetrician-and-gynaecologist