Please help!!!

Please help! I went in for my first ultrsound last week, and the nurse called me and said I'm 16 weeks judging by the babies size, but my LMP was march 13th-16th, if I'm 16 weeks that would mean I got pregnant around april 15th, which is when I missed my first period. It doesn't make any sense! could something be wrong with the baby that would make him so small? Could I be further along than the doctor's think?