What Causes Delays In Menstruation?

Hi, i am 26, and i am sexually active with my boyfriend, last three months periods were almost normal, lasted for 3 to e4 days.. but this month its late by 16 days. and i have suddenly put on 9 kgs, i get back ache and head ache very often. i susspected a pregnancy so took 6 hcg home test.but all negative results. finally i consulted a deoctor. he again took my urine sample and result provede i am not pregnant.he gave me primolut-N. i want to know whether i shud take this medicine or still confirm?