It's hard to say although it's not unusual for periods to be erratic / irregular the first few years of menstruating. Did your doctor not say anything about the lack of periods when your hormones were checked? It may just take a little more time for your body to get into a monthly pattern. And one can sometimes have cramping without having a period. But then cramping isn't always gynecologic related either.
Also, there are endocrine or metabolic conditions that can cause irregular cycles such as thyroid, pituitary, adrenal, polycystic ovary syndrome (insulin resistance). A workup may be in order if periods continue to be absent.