What Causes Delay In Menstruation Other Than Pregnancy?

I've never had a late period. It's always been like clockwork for me. Then 8 months ago I was 2 months late and then 3 months late again and presently, I'm 2 months late and counting. I'm worried because I'm not a stressful person, I've always eaten healthy, and I have always worked out. My lifestyle has not significantly changed and I weigh the same as always. What could be the problem?