Possibly twins

Hey ladies. Quick question. I am currently 15 weeks. I haven't had an ultrasound since 6 weeks. We saw the flicker of a heartbeat but no sound. Then when I went a week and a half ago for the Doppler heartbeat we heard a heartbeat. Twins run on both sides of my family. I am nervous with as big as I am for 15 weeks and it being my first that Twins could pop up on dec 7th when we go for our sonogram. Has this ever happened to someone? Only see (very early) and hear one but later see two?