Undetected twin???

I had my first ultrasound at 6 weeks and it was revealed that Im carrying 1 baby. Today I went for another ultrasound at 11weeks 5 days and the doctor told me Im having twins!!  Is it possible that one baby might have been hiding or too small to be detected or behind the other when I had my first ultrasound???Nevertheless it was a HUGE surprise!!! Me and my husband are soooo happy!!!!