Flat spot ???

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So my little guy is 3 months old and up until about 3 weeks ago his head was FINE. Now he will only sleep with his head flat (I used to turn it to the side, alternating sides each night). But now every time I turn his head he turns it right back to being flat. I’m SO worried he’s going to have a misshapen head and need a helmet. Any tips to prevent that are greatly appreciated