one sac one yolk 2 babies?

is that possible? I've only had an ultrasound at 2-3 weeks. I had it that early because we thought I was farther (8w 5 days based on lmp ..I ovulated late) i am so big already and I can feel my uterus and it's already up well into my belly button. I'm 15 weeks 2 days and can feel movement. it is my second but what you guys think? Or am I just crazy? I never had a follow up ultrasound because I switched doctors. now I won't have one until 22 weeks.