Im approx. 4-8 weeks along, had ultra sound , seen gestinal sac, but no baby. Is something wrong?

I missed my menstal in May. I cant remember when I had it in April, but i know it was like for only 2 days in the beginning of the month, which i usualy have it for 4 days. I do usualy write it down but I cant find my calendar for March, April. I did a preg. test 2-1/2 weeks ago found out I was prego. I went into clinisc did a test just to certain that following week, they scheduled an ultra sound to determine how far i am, and because i feel bubble, butterflies, type feeling movements. So i went in today had an ultra sound , they seen the gestial sac but no baby inside.They mesureed the sac at what would be approx. 6 weeks along. So my dr. called me wants me just a little bit ago to come in n do some blood work and on monday. Im worried, this just does not makes sence to me. I hope everything is alright. I have a six yr. son, my pregnancy whith him was all normal, although I didnt find out i was prego untill 3 1/2 months along. this will be my 2nd pregnancy. Does anyone know about something like this happening to someone they know, or themselves, and should I be concerned??