neck pain upon waking

What could be the cause of back pain?
I have been waking up every morning for over a year with back pain. I wake up stiff, unable to move. It goes away after 20 minutes or so of sitting up. What could be causing it?
It’s not my bed. My bed is brand new, less than 4 weeks old. this back pain started long before I got a new bed and did not stop after getting a new bed. Also, I’m not overweight either, I’m at a proper weight for my height. I’m 28.
My old matters wasn’t saggy. It was still in good shape, just a few years old.
No, I don’t sleep in any weird postions, just on my side. I’ve slept on my side for years upon years. It wasn’t until 14-16 months ago when all of a sudden I started waking up with back pain. It came out of nowhere.
I do not, will not wear high heels.
The only drugs I am taking is prescriton ibuprophen, which my Dr. prescribed me to deal with my back pain.
My neck is fine. My spouse slept on our old bed with no problems, and has had no problems with our new bed.
do some lite excise and stretches before you go to bed like warm downs you will feel 100% in a few days
Hatha Yoga for Neck and Shoulder Pain – 57 minutes