If you are young, and it sounds like you are, your periods can be irregular due to shifting hormones. If you are sexually active, or your period does not stop, you need to visit your doctor.
I had my period last week. Was nauseous afterwards for days. Along with lightheadedness. Then the next Friday, I started bleeding again. What's going on?
See a doctor. You can see your family doctor, if you don't have a gynecologist. What you are describing isn't normal and excessive blood loss can lead to anemia.