What Causes Irregular Periods With Bad Cramps?

Hi, I am 18years old and I attained puberty at the age of 12. Initial years, I had no problems, may be after 2-3 years I started with irregular periods like after 21days i can get anytime before the completion of 30days. Also, I get very bad cramps and i start croaking from the moment i get my periods. THis croaking continues on first day alone. Please help me!!!!!!