What Causes Missed Periods?

My name is Kelsey and im 19 years old. My husband and I started trying to concive in october of 09 and we are still not pregnant. I have always had irregular periods, but since we started ttc in october I have had a period in october and then in January. Those are the only two periods I have had. Since we have been trying for a while I am starting to ge worried that my lack of period may be the problem. Could this be possible? What do you suggest I do?