Okay i really need help!

So my friend gave me a pocket fetal doppler. I have learned how to use it and recognize the difference between the baby and I. Now that i am futher along (14w 2d) I love to hear the heart beat! But for the past days it seems like i hear overlapping of the baby heart beat and if i spot one single heart beat  and move to a different spot i hear another one.... This makes me think that i have two insted of one. My first U/S was at 11w 5d and it showed one baby. My next U/S is at 20w. Could it be possible that i could have twins and they didnt see it in the first U/S? Also can i ask them to check before my 20w U/S?