period always late

my periods always late. always. there has never been a month where it hasn't been more than a week late. in March of this year it was 2 months late. I'm 16, and really underweight (I can't help it and don't starve myself, I eat normally). I'm 5'6 and about 98 pounds. could my weight be the cause of this? as of now my periods one month and 2 weeks late. it usually ranges from 1 week late to 2 months late. I also am sexually active, so you can imagine how freaked out I always am. should I see a doctor about this? or is this normal?