A friend of mine that is a massage therapist/physical therapist taught me an exercise for my lower back and lower stomach.
Lie on the floor with your knees bent.
Raise your legs with the knees still bent so that it looks like you are sitting in a chair.
Than lower your legs in this same position, feet flexed, aiming your heels towards the floor. Only go as far down as your stomach muscles will allow.
Than raise your legs back to the starting position.
I started out only being able to do about 5 because of my back and stomach. I can now do around 30 at one time. I usually do sets of 10.
But be nice to yourself and don't stain too much.
I know this one works on the lower stomach muscles but it will also strengthen your back. To me it is a win-win