To really know the origin of the problem you would have to have your doctor do a full hormonal panel on you. It is a blood test and will tell the doctor if your hormones are at adequate levels.

Sometimes doctors can not tell you why you are suffering from amenorrhea. I had a similar situation in my early 20's. I did all the testing, had hormone shots to try and get my cycle back on track. Nothing worked. I finally went to see an acupuncturist. It took a few visits but it worked. You may want to consider this option.

Yeah, I would see a doctor. Are you eating well, getting enough rest? (those are the easy fixes) otherwise there might be some medical cause.