What are various reasons for having late or missed periods- but you're 100% positive you're not pregnant. ?

Missed period: Lots of things can cause women to miss or have a late period: Stress, illness, medicines, hormonal fluctuations, rapid weight loss. If it becomes a recurring problem then it's best to get checked out by your doctor.

Stress, weight loss.: It is not uncommon for young women to have occasional changes in their menstrual cycle. Next to pregnancy, stress is a common cause of missed periods. Other reasons include weight loss, excessive exercise, & illness. Do you have a reason to be concerned? - Unprotected sex since last menstrual period?