Pregnant but no gestational sac... worried

My LMP was March 23, 2012, I went on vacation with my husband and got back on April 16, 2012. I took a pregnancy test urine and blood on the 19th and it came back positive! We are excited that on our first try we were able to conceive. My BHCG level was 8.63. I retook the pregnancy blood test on the 27th and it was 403.96, i went to the ER for constipation on the 29th and my bhcg level was 976, they did a transvaginal and transabdominal sono of the pelvis but no yok sac or fetal pole was found. Should I be worried? or maybe is it too soon to see the sac???? Not sure what to think