Ultrsound Finds 2 Sacs, But Can Only See 1 Baby

Hi! First of all let my start out by saying I had a feeling I was having twins almost immediately. I went for my first ultrasound appointment last week to find that I was 6 weeks 6 days pregnant on that day. The Dr. also showed me that there were definately 2 sacs, but explained because the way my uterus is tilted they could not get a good view of the second one. The first sac was very visible and we were able to get a good look at the baby. The second sac, even though we did an internal ultrasound, was just too hard to see. The doctor said that I could have already lost that baby, or it could just be too soon to see it and we won't know til my next appointment, which isn't for another 3 weeks. I am worried to death! Has this happened to anyone before? I know that a lot of twins are missed this early in pregnancy, but how many times has one not been seen in its sac?