What Causes Missing Periods, Cramps And Fatigue?

I am 23 years old, and recently married. We went on our honeymoon, and two weeks after we got back I had a menstrual cycle. Since then I haven't had a period, but have had faint cramps like I usually do before my cycle. In the last two weeks I have become very fatigued, weak, food is not appealing and nothing seems to smell right. I have taken 3 HPT and all come back negative. Could I be pregnant or is something else going on?