I have always had regular periods since 13 y/o but am pretty healthy. Stress on the body such as illness can stop periods (Most elite sportswomen don't have periods when training because of high stress to body). Basically, if the body feels it cannot sustain a pregnancy it will not ovulate. Keep up your nutirition and exercise moderately and it may come back.
Good luck - I wish I had you problem lots of the time! AS for PMS, I get it REALLY bad - i am a nasty person 2 weeks per month unles i am on the pill. The hormones in some oral contraceptives really help with reducing cramping and mood swings closer to the period and also seem to lighten the flow.