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The absence of periods can be due to hormonal
imbalance which may even be caused by the pills and other causes like thyroid
problems;PCOS;and even pregnancy
can make you miss periods.Since the pregnancy tests you have done are negative then it may not be pregnancy and may be some other reason which is causing absence.So please consult a Gynaecologist and get investigated like get a ultrasound
done to find the reason and if needed blood investigations.After proper diagnosis the Gynaec will treat you.