It's common to have irregular periods once in a while. But if it continues, you should visit a gyno clinic. A couple of people mentioned stress , hormone changes, BC pills, can cause period irregularities. I had two 8-day periods in one month.

Anemia is the result of an underlying disease, blood cells carry oxygen, so you would feel very very tired, depending on the severity and especally on your period. It may be other way around..heavy periods can cause anemia in many women and it does so. the most common cause is diet. Even tho we don't have signs of anemia until the iron storage is depleted, however iron isn't easily absorbed, especially if you take it with food/milk and if you have a genetic anemic disease. Vit C helps with absorption (ie green tea with lemon). What event suggested that you have anemia?