Thanks for your question.
You did not mention your age which is important. If you are in perimenopause, you will have menstrual irregularities. You also may just be having one period that is unusual. Most doctors, after checking you for pregnancy
, would say to just 'wait and see' if the same thing happens next month.
If you are not pregnant and not perimenopausal, the reason is simply irregular estrogen
levels. It is the balance between estrogen and progresterone that cause the monthly cycle. Just before your period, both drop abruptly. This is what causes all the usual premenstrual symptoms followed by your period.
If you are pregnant, you will need an ultrasound
to check for an ectopic pregnancy.
Hope this helps,
Carol Rogala d.O.