Doppler advice please

Ok so..im 12 weeks i had a scan at 8+6 that showed one baby. I bought a doppler and ive been picking up a heartbeat for a few days now on my left side.. Something made me move the doppler over to my right side tonight and straight away picked up another heartbeat!!! If its twins why wouldn't they have showed on the scan? Or is it just sound travelling or something? Any one else experienced this?