Hidden/vanishing twin?

Went in to the ER for some spotting at 13 weeks 3 days and they did an ultrasound. The doc asked me if I was having twins and I said no!? She looked around and pointed out that she kept seeing this pop up behind baby but couldn’t make out what it is. Thought maybe a vanishing twin? She also said the way my amniotic sac was moving around baby is the way it looks with twins but I’m not sure what she meant. Any insight!?