over 6 weeks pregnant and no fetal pole or yolk sac

I'm 6.5 weeks pregnant. I went for an ultrasound at 5.5 weeks and the doctor didn't see anything. A few days later, my pregnancy symptoms have diminished and my lower back is in severe pain constantly. I went in for another ultrasound and still no sign of fetal pole, yolk sac. The doctor wants to see me in 5 days. I do not "feel pregnant" anymore and his order of diagnosis is 1. missed miscarrage 2. twins that are not visible yet 3. normal pregnancy or 4. etopic. If my levels were at 11,000 and didn't double but went up to 19,000, shouldn't I have seen something on the ultrasound screen????Any help would be appreciated. Thanks