I Have Missed My Periods And Is Already 5 Days

i have missed my periods and is already 5 days from due date, mine is 26 days cycle. took a pregnancy test with kit available at medical store, result is negative. visited doctor yesterday, she prescribed sysron n for 3 days and on 4th day nuforce kit 3, my periods were never irregular for past 10 years and i m married from last 6 months, i m obese and my age is 25 years. I would like to know what exactly is the use of medicine.