What Causes Delay In Periods And Tightness In Lower Abdomen?

Age: 24 Ht: 5'3" Wt: 120lbs I have had 2 children. I was supposed to start my period on the 21st of last month but I still have not started. I took a PT that was negative. For the past 2-3 days I've been having tightness in my left lower abdomen, accompanied by dull/cramping pain and occationally severe shooting pains. I have been suffering from fatigue. No matter how much I eat, I continue to have a gnawing feeling in my stomach and since yesterday evening, I have not wanted to eat at all. I do not feel nausious-just cramping or burning, but when I try to make myself eat, I gag or feel the urge to gag. What are some possible reasons for this? FYI: I did take a morning after pill around the 5th of last month and there is a possibility that I could be pregnant.