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 :)
Very happy with the results, I incorporated my own food tastes and mixed them with lots of veggies, fruit, kale, spinach and romaine lettuce, a huge salad everyday, no sugar, no white flower, no bread other than 100% wheat a couple times a week, cut out soda entirely. Greek yogurt every day, berries, only sweeten with honey. Old fashioned rolled oats, etc etc. I didn't use any of the recipes, too complicated, and costly too! I have lost 30 lbs from January 5th to now. I didn't exercise the first 2 months wanted to make sure I was losing from the food. I then started walking 2-5 miles 3 times a week - 15 pounds to go to my goal.