Negative pregnancy test no sex in last 4 months but menstral is unusually 2 weeks late, what could this be due to?

Some possibilities: In a woman with regular periods, some causes of a late period include: excessive weight loss or weight gain, increased exercising, emotional stress, having an illness, traveling, medicines (like birth control pills), hormonal imbalances, etc… Seeing a doctor is a good idea if symptoms are persistent, recurring, or worrisome. (pregnancy is the common cause of a missed period, if sexually active)