Hello dear Understand your concern. I think it is due to hormonal imbalance as a side effects of Plan B Plan B tablet is emergency contraceptive pill contain 1.5 levonogestrol. It inhibits the ovulation and prevents the pregnancy. It is highly effective if taken as early as possible with in 24 hour of sexual intercourse. 95 % effective if taken with in 72 hour of unprotected sexual intercourse. So may be chance of pregnancy is 5 % You should go for Blood HCG or abdominal USG for better confirmation of common side effects are below irregular/early menstruation due to hormone imbalance Bleeding between periods or heavier menstrual bleeding Nausea or vomiting Dizziness Fatigue Headache Breast tenderness Lower abdominal pain or cramps Also note the other cause of irregular period: 1. High level of stress 2. Change in life style 3. Excessive exercise 4. Thyroid problem 5. Hormonal imbalance Also note that Menstruation required co ordinate action of various hormone that secreted by ovary (estrogen and progesterone) and brain (FSH and LH). If any above hormone is decreased or increased there is imbalance in menstruation. So please do full hormonal profile : Estrogen, progestron, LH, FSH, HCG and thyroid hormone. That help to diagnose hormonal imbalance. After confirmation you can take treatment according So please go ahead. Hope this may help you All the best. Stay healthy.