Follow up ultrasound???

Catlin
I just went for an ultrasound today and thought everything was great. The baby's heartbeat was 160 bpm and the ultrasound tech said the baby was measuring right on time. 
But the doctors office called and said my doctor wants to do another ultrasound to check viability again in 2 weeks ( I will be 9 weeks and 6 days) 
Any thoughts??? 
Now I am so nervous. I don't know what to think.