I think it’s totally normal. It’s just because your abs are not strong enough yet, so the rest of your body tends to compensate for it. It does the same for me, though maybe not as much as you said. But I think it’s still perfectly normal! Just keep going and one day you’ll notice that you arch less. Also, I think everyone has a space between the floor and their back when they’re lying, depending on your butt’s form :)
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