6 weeks pregnant With twins but found a collapsing sac!! HELP

[Lovingmommyxo]

So, I went to the hospital the other night due to vomiting and stomach pains.. I found out I was pregnant but the hcg levels were low.. So they have been checking up on me for 2 weeks now.. And my levels jumped from 21, to 54, to 1,888. But I had an ultrasound done im 6 weeks pregnant and my doctor said they seen a " collapsed blood sac" but I do have one baby that is okay. So I'm not really understanding if I'm having twins? If the collapsed sac means ill miscarry? Or is there a chance the 2nd baby is okay? I'm so confused and worried.. I need answers please!!