Sometimes when i bend over, a sharp pain shoots up my lower back for a second, almost crippling me and making me fall over, what's going on?

Disc disease: This could possibly (among other possibilities) represent a bulging disc. Discs most commonly bulge postero-laterally (back and to the right or left). Bending forward will tend to cause the vertebral bodies above and below to sort of "squish" the disc thereby aggravating the bulge. This could increase the degree of stenosis (narrowing) in certain areas allowing for nerve impingement.

SI joint: Sharp back pain from bending over is frequently cause by si (sacroiliac) joint dysfunction. Disc dz of the spine usually travels down a leg when the disc irritates the nerves. Lumbar arthritis can also cause a lot of mechanical lbp with movement. Muscle spasms of the lumbar spine can be so severe as to "lock you up" and make it hard to move. Usually treatable with therapy, meds, exercise.