AIBU to want to know why my lower back hurts every morning?

[Thingsthatgo]

Does anyone else have this? I am in my forties, fairly fit, normal BMI. Every single morning my lower back hurts. Just for 5 minutes or so it feels like I can’t straighten up properly. Is this just how life is now?
Maybe I need to do some yoga or something?
I also can’t sleep on my side without a pillow between me knees... is that normal?

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I get it a lot, though not consistently enough for me to blame the mattress, which is fairly new and expensive. A short walk with an old dog first thing usually sorts it out, then a hot bath to iron out the rest of me. Fuuuuck, I am decrepit.

Magnesium.... that’s interesting. I get weird restless legs as well, and sometimes use a magnesium spray.
Thank you for all of the suggestions, I shall try them, and see if anything helps.