Im not pregnant. blood test negative. but i missed my period for the first time ever. why?? should i be concerned

It happens sometimes: In a woman with regular periods, some causes of a late period are: 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 a common cause of missed periods, but not if negative tests.