no sac seen in ultrasound at 5 weeks.

My lmp is nov. 28. I missed my period last dec so i got a pregnancy test through blood and the result is positive. Im. So excited so i got my ultrasound jan. 7 to see if the my pregnancy is ok but to my surprise, nothing was seen on my uterus. Am i really pregnant? The ob said it might be very early. Got a test again after 2 wks.