welcome to HealthcareMagic
The irregular periods
can be due to PCOD( cyst in ovaries);hormonal
problems;mental stress etc and even pregnancy
can cause to miss the periods.If more than 2 pregnancy tests are negative then you need not worry about the pregnancy and the sick feeling may be due to some other reason for which you can consult a doctor in person and get examined as it can be due to gastritis;mental stress;being overworked;anaemia etc.