Hiding twin??

Is it possible to be pregnant with twins and the doctor not catch it?
I have an at home doppler and most times can pick up what it sounds like is two heartbeats, also i can feel baby kick with my hand, I'll have my hand on my very right side of my stomach and feel a kick then immediately after more kicks on the left side and so on.
Only one baby was seen on my 12 week ultrasound, but I've heard it's possible for a twin to be hiding behind the other.
Your thoughts??