sonogram not showing anything ...

Judy
Two days ago i found out i was pregnant. The clearblue digital show pregnant 1-2... That means i am 3-4 weeks along right? I went to my obgyn and she did a sonogram fof me and said she didnt see anything. Should i be worry ???? Or is too early too see any thing (even sac) for 3-4 weeks pregnancy??? Need some re-assuring :(