I'm on the same boat as you! I've missed 3 periods and have felt nauseous, tired, bloated, gained weight, etc. My doctor tested me for anemia, thyroid problems, and pregnancy and all tests were negative.
Stress is the first thing that comes to mind but it could be a hormonal imbalance, ovarian cysts, age and other medical reasons... definitely need a gyno's advice on this one!
GOOD LUCK!