You may be correct when you wonder if the mattress is responsible.
Many people experience a bad back just a few days after sleeping on a softer surface than they are used to.
Clearly, movement is good for your back and I would continue the exercises but I would emphasise the Yoga position known as "the child" here in the west, and another where you squat with your knees tucked into your armpits.
Breath deeply and allow the expansion to stretch your upper back, pulling the shoulder blades apart.
Finally, try some forward and backward shoulder rolls but really exaggerate the movement.
Your mattress may not be supporting your back at all well, which is not good for several hours at a time.
I hope this helps