If you used the RNP did your LO develop a flat head? If so, how long did baby sleep in it & when did the flatness go away/LO's head round out?

Sadie is almost 8 weeks. She sleeps in it every night & for an occasional nap. She naps in the swing, in my arms, or on her boppy usually. Most of her awake time is flat on her back. She has developed a crater-like indent on the back of her head. It's not flat, but there's a large circular indentation. I'm worried - don't want her to have to wear a helmet!