Going that long without menses is not normal. It is almost always because of some hormonal influence, and as such, to sole it you need at least a blood test and an exam.
Now, hormone variations can come from many things. For one, excessive exercize or being to thin/skinny can stop your periods completely. Various illnesses can do this as well, but some are not expected in young women. Polycystic ovary syndrome has already been mentioned and can indeed occur for anybody. Also something called endometriosis can cause problems (also results in rather painful menses). It can also be a problem in the gland that regulated the hormones, a gland called the "pituitary galnd" it sits right up under the brain and also regulates hormones for growth and for your thyroid. So if you have weight problems and signs of an abnormal thyroid (especially if your mom, dad or others in the family also have/had those problems), then that needs to be looked at.
But for the physician to just ignore it as not important is not appropriate. There could be numerous reasons, some of which need to be looked at, and some of which may also affect fertility in a rather significant way.