What Causes Lower Back Pain, Nausea And Lightheadedness?

I am a 47yoa female. As of a week ago I had not had a period since Aug. 31, 2010. I took I had cramps at time my period should have started but no period. Over the past couple of weeks I had lower back pain, nausea and lightheadedness. I took a pregnancy test but it was negative. Last Sunday I started having severe sharp pains in my lower abdomin and at one point I saw a light brown-pinkish discharge. After that I still had sharp pains all nlght but no other bleeding. The next morning when I went to bathroom as soon as i sat down I felt something come out of my and hit the toilet. I sank all the way down so I didnt see it. After that I started bleeding and had severe cramps and lower back pain. The bleeding and cramping lasted 3 days. What could I have passed?