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The normal cycle of periods is between 21-35 days anything above or below this is not normal.In young females the irregularity of periods can be due to hormonal
or any other cause which needs investigation.Since you said the reports of ultrasound
were normal that goes to say that there is no internal organ problem so it can be hormonal which can be treated with medicines if there is a need to do so.As for the pain in abdomen during periods it is a common problem faced by all females till there is a vaginal delivery
and it will decrease post marriage ;till then you can control it with antispasmodic medicines.
I hope this answer is helpful to guide you .