10 weeks no heartbeat

So I went to the doctor yesterday and they told me that one baby had a heartbeat but the other didn't. The one that didn't is my smaller one that measures about a week behind the bigger one. But last time I was at the doctor they both had heartbeats. I have not had any pains but the doctor told me that the small one would be a miscarriage or disappear. She gave me no hope of it just being turned wrong or anything. I am so confused. They told me the reason one was smaller was two conceive dates.  Anyone had anything similar