What the hell?!?

Hi ya ladies,

I ask currently 18 weeks with 1 and tonight I checked my little ones heart rate with my hospital grade fetal heart monitor and heard something weird. I could hear 3 heart beats. My babies, mine and then I could hear another heartbeat on the other side of my uterus that was just as fast if not a little faster than my baby's. I checked my pulse and it was not it, I know the placenta makes a swooshing sound so that wasn't it. What is the world could I have heard. This is a first considering I listen every night.