Could i have an undetected twin?

I went in for a 8 week ultrasound and they saw 2 sacs but found only one heartbeat. I went back in for another ultrasound at 17 weeks and 4 days. To me it looked like there were still 2 sacs when she ran the thing over the top of my stomach but the tech said there was only one baby. Is it possible to carry 2 sacs and only one baby through a pregnancy or maybe the other baby is just a really good hider?? I am so confused. My doctor is scheduling another ultrasound that he, himself will be doing. What do ya'll think?