Could this mean I'm having twins?

I'm about 16 weeks pregnant now (with baby #2) and haven't had an ultrasound. My doc won't give me one until I'm about 20+ weeks. I've had 2 check-ups so far and my 3rd one next week. Well, the last time I went in at about 12 or 13 weeks, the nurse who doesn't normally try to find the heartbeat with the doppler went ahead and tried to do it herself and she found a heartbeat kind of close to my belly on in the 120's and she was like...hmm that might be the baby. Well, my doc came in and took over and she definitely found the heartbeat really low bellow my belly on and it was in the 150's. Could I be having twins, or was the nurse picking up my heartbeat? 120 sounds high for an adults heartbeat. I didn't have morning sickness and I think I'm showing normally so could it still be twins, or would there be more signs. Thanks