One exercise a physio gave me to help my back was to get a mat and a very very LIGHT dumbell.

Go down on two knees and one hand with your back horizontal to the floor.

Holding that alignment very carefully lift the dambell up out to the side and lower it again slowly.

You do it 2 ways:

1) straight out horizontally level with your shoulders,

2) also diagonally from under your left hip up to your shoulder level out to the right.

Do both sides too.

It's really important to use only very light weights because it is to work muscles which are small right in the middle close to your spine.

You won't feel them or work them correctly if you use even slightly heavy weights - you'll end up working the wrong muscles which won't help.

Also a good back release I enjoy is to lie on my back with arms out to the side (passively stretches the chest), legs up at right angles with thighs resting on a chair. Ten mins or so of this when your back is tight can make a big difference.

Hope this helps your back - i've got scoliosis which makes my back a weak point that gets inflamed occasionally. These exercise help me.