What Causes Missing Periods And Negative Pregnancy Test Reports?

hiii...i am 22 yrs old. my periods are 12 days late today. but pregnancy report is negative. i feel very tired. backache. even i am thyroid patient. doctor suggested me to take medicine so that periods shud come.my periods were regular.never happened this earlier. but i dont want to take medicine for periods. what should i do?? pls help me.