How do you know the difference between period blood and bleeding caused by something more serious?

Periodicalilty: The period is periodic, will last few days, stop for about 3 weeks and repeat the cycle.

First time?: Period bleeding comes on schedule and stops after 4-5 days. It may begin as bright red, and finish in darker colors. Dangerous bleeding is unpredictable, ongoing, persistently bright red, and with symptoms like weight loss, nausea, light -headedness, or fainting..