I get a sharp pain on the lower back and it throbs, pulsating with the heartbeat. it throbs going up the spine and i can feel my whole back up to the neck throbbing painfully. this usually happens when i'm startled, causing my heart to beat fatser.

Need to examine: It is not feasible to provide a meaningful opinion without taking additional history, physical examination and may be some tests. It would be prudent to see your doctor and discuss if you may be depressed. Counter-intuitive as it may seem, exercise may help. Start with low intensity exercise such as walking and gradually increase the intensity as tolerated. Wish you good health!