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can be due to hormonal imbalance;ovarian cyst
also.The problem is if you had irregular period and did a unprotected sex during the month there can be chance of getting pregnant so if the urine pregnancy
test is negative you can do a confirmation by doing a blood test
for HcG and if that is negative please consult a Gynaecologist for examination and treatment.My advice is do not take Cyclenorm P &E or any other medicine without advice from a Gynaecologist because it may further complicate your irregularity of menses.Please consult a specialist.