Hello dear, Emergency contraceptive pills like next choice have a high hormonal content. When taken, they cause hormonal imbalance in the body...which can explain the delay in your periods. However, other than pregnancy, delay in periods can also be due to many other causes like hormonal imbalance(as already explained), anovulatory cycles, stress, any infection or pathology in the uterus, ovary, etc. So, if pregnancy test is negative & periods are further delayed, kindly consult a Gynecologist & get a proper clinical examination done. Investigations like complete blood count, Ultrasound scan of abdomen and pelvis will be helpful to rule out any pathological cause for your symptoms. Wishing you a good health. Take care.