Lower back pain while laying down

I am a 21 year old female, with a slight spinal curve to the right (but very slight so my GP has told me it shouldn’t be a problem).

I walk a lot and do Pilates consistently, and I’ve noticed recently that when I lay down on the mat when we do ab exercises, my lower back experiences a lot of pressure on the floor. It’s almost as if my lower back protrudes more than my upper back which causes friction?

I have occasional lower back pain but it gets better when I do core exercises consistently, but this pressure I’ve never felt so strongly before. My GP told me that it could be friction from the mat with my bones since I’m quite slim…. I also feel light pain (friction similarly) in my coccyx as I roll down from crunches for example.

Do you think this is something concerning or just physiological thing to be aware of?

Thank you so much!