What Causes Two Periods Within A Month And Stomach Pain?

Hello, my name is Jessica I had my period a week ago it was the normal heavy bleeding And today I woke up to a period again it was new and old blood mix together and lower part of my stomach hurts and a little worse than cramps I am 18 olds old and no kids I am sexually with one partner. And this is the second month this has happen to me. What does this mean? Thankyou!