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If you have missed periods and sexually active the first thing to rule out is pregnancy
.So if you are having morning sickness
but if pregnancy test is negative then you can get a blood test
to rule out pregnancy and even a ultrasound
for confirmation.There are other reasons for irregular or missing periods like PCOS(cyst in ovary);hormonal imbalance;thyroid problems etc.
Please consult a Gynaecologist and get investigated and treated.