Is it possible for ultrasounds to miss twins?

I'm 6 months pregnant. and no I haven't seen two babies. It's more a feeling. I also went to a gender-check when I was 20 weeks..they said girl. then 10 days later I had another ultrasound and they said boy. I have been extremely nauseous and tired with this pregnancy. so much so that I have lost 10 lbs. but my belly is measuring 3 cm big.. but the baby isn't. He actually only measured in the 32% at my 21 week visit. I felt movement when I was 11 weeks preggo.they told me it was gas but this is my 3rd pregnancy i know better. Now I can feel movement up by my ribs and down at my pubic bone at the same time. should I ask for an afp test or an hcg test? Or just talk to my doctor and see what she thinks? its really weird. I don't go one night without dreaming its twins..and my MIL called me this morning and said she had a dream that it was twins too..one boy one girl. lol.. i'm just wondering if its possible for there to be a hidden twin.