Hi and thanks for your query, the commonest cause of lack of normal periods in a female of reproductive age is pregnancy. If you have done a pregnancy test, of good quality, under the standard conditions of a urine sample in the morning, and it negative, then its ok. Females early in their reproductive years might experience cycles without any bleeding usually called anovulatory cycles, and this is but normal. But however, if you fail to see your period again next month, or start experiencing weakness, tiredness or vomiiting or morning sickness, you might need to run another pregnancy test. The gold standard test to do to be very sure, if you want at this stage is doing blood levels of Beta HCG in blood or doing an obstetric ultrasound to clear out all doubts. Remember the pregnancy strips at times might have false positives. Thanks and best regards, Luchuo, MD.