What Causes Painful And Irregular Menstrual Periods?

I had gotten my first period at the age of 9 and 8 months. I am now 15 years old and have irregular menstrual periods which began at the age of 13. I would get it twice every month for a couple of months and then none at all for one to three months. When I do have it, the cramps are bad enough to leave me bedridden for the entire week. What could possibly be the cause? Are they symptoms of a more serious illness?