What Causes Missed Periods Without Being Pregnant?

I have not been getting my periods like I should be. I missed my period for two months and then got it and assumed that it was going to come back like normal. Then it disappeared again for three, going on four, months. Should I be concerned? What are some possible causes? Should I see my doctor? I'm nineteen and started my periods at the age of twelve. I've always been regular. I'm not pregnant and I have no other symptoms.