Flat head

So my 2 month old has developed a flat spot on the back of her head at first it was only the left and now it’s like the whole back of her head is flat... her pediatrician tried to turn her head to the left and said she has a bit of stiffness. (She always favors the turning her head to the right when she sleeps at night) even if I try to move it. I really don’t want her to have to wear the helmet.. but I don’t understand how else I can fix this when they say absolutely no laying them on their side of stomach when sleeping, back only. During the day I don’t really have her laying down on her back she’s either laying on my chest or on her side while I’m watching. Has anyone been able to fix this if so what do you recommend?