Lower back stiffness when sleeping on my side

For the last couple of months I’ve been waking up with this annoying stiffness in my lower back, especially on mornings after I’ve slept on my side. It’s not really sharp pain, more like my lower back and hips feel tight and a bit out of place when I get up. After I move around for a bit it usually goes away, but it’s been happening pretty often.

I thought it might be my mattress at first, but it’s quite new and pretty supportive. I also tried putting a normal pillow between my knees since I’ve heard that can help with alignment, but honestly it just ends up sliding away during the night.

I was looking into it I randomly came across something called the Buttress Pillow and from what I understood it’s supposed to stay between your legs and keep your hips and lower back aligned while you sleep. I had never really seen a pillow designed specifically for that before, so I’m not sure if it’s actually useful or just clever marketing. The shape also looked a bit unusual compared to normal knee pillows.

Has anyone here tried something like that or found something that actually helped with lower back stiffness from sleeping position? I’m starting to think it might just be an alignment issue but I’m not really sure what the best fix is.