Yes this happened to me with my first baby. Ultrasound at 8 weeks showed 2 sacs but only one had a baby in it, the other was empty. It was explained to me as "vanishing twin syndrome". The empty sac measured at about 6 weeks when the "miscarriage" was thought to have happened. However i never spotted once through my entire pregnancy. Apparently the other baby and myself absorbed the embryo. My son was born completely healthy and I had an amazing pregnancy. My husband likes to tease and say my son ate his twin.
It sounds like as if now you are having twins. Drink lots of water 80oz or more a day, and take it easy as much as you can. Nothing you do will change the outcome. I have twins, and it is hard work, but worth every single second.