I Am A 40 Year Old Female. For The Past

I am a 40 year old female. For the past 2 months I have noticed when I lay down at night on my left side I notice a change in pattern of my heartbeat. It feels as if it flutters or skips a beat. I always have to roll back over to the right side which seems to not bother me. I don't notice any heart irregular during the day or normal activities. What could cause this to happen?