What Causes Missing Periods And Abdominal Pain?

hi i have been on the hcg drops for a month now, while taking the Dianne 35 contraceptive. i finished the drops about 7 days ago and am supposed to see my period today. I am feeling all the pre menstrual symptoms, eg. abdominal pain but no bleeding. I also missed my period last month. I know i am not pregnant because i did a pregnancy test. Could this be serious?