What Can Cause A Missed Period?

I've had three missed periods in the past three months. I took pregnancy tests but they all came out to be negative. For about a month now my eating habits have changed due to a diet of just eating healthier because I thought I just was not eating right. For about two weeks last month I'd feel extremely nauseous when I eat after I eat even when I don't eat. I noticed I've been drinking lots of water so I thought it was dehydration. Lately I've started to feel extremely weak and suddenly tired. What is wrong and what can I do to make myself better?