Nervous after my doctors appointment

Hannah • Twin girls born 6/26/15
I went in for my 12 weeks visit at 11 weeks and 2 days and when he was listening to the heartbeats he only found one and it's making me nervous! My next ultrasound is a week from today but it's making me nervous that there is something wrong with one of them. He didn't seem concerned which is weird to me. Anyone have this happen?