Empty sac

Went in to the ER for some cramps at 5w6d and doctor saw a 5 week fetus and an empty sac, looks like I had twins but one didn't make it. She said I would bleed and would lose the empty sac but nothing so far and it's been almost a week. Now I'm wondering if that empty sac was not empty and just too small to see. Anyone else have an empty sac that needed up being ok after some time?