Found twin later?

Amy
Any of you ladies have your first ultrasound and only see one? And then found out later that you are having 2? My 8 week ultrasound shows one baby, which is a blessing but today with my Doppler I heard my own and two other heartbeats, the same one I've heard every other time and now a vastly different one. And now 
 'm freaking out and seeing a second yolk sac in my first ultrasound.....