Flat head

So my husband and I have just started noticing that one of my twins has a flat head on her right side and the left side is fully round. I noticed she favors the flat side and always wants to be laying on the flat side, she always looks the way of the flat side etc... I read online to keep turning her head away from flat side so she learns to lay on both sides etc. I'm a little nervous though has anyone else experienced this? Will it round out? Is her brain ok? Why does she favor the flat side?! Hellllllp! I will ask pediatrician at their 4month appt next week. Thank you!