Since you have a pacemaker, it could potentially hinder you from doing a lot of strenuous exercises. However, there are some simple exercises that are allowed. Having said that, before doing any exercise routines, you have to have cardio clearance from your Dr. Please check this link for easy low back pain stretches.
I also suggest putting hot packs over your lumbar area every morning for 20 minutes to address morning stiffness as heat relaxes muscles and joints.