Lower back spasm which came up after working in the backyard followed by lifting your grandson points towards a muscle which went into a spasm because it lacked the strength endurance to do repeated tasks without going into fatigue which is typically seen persons with reduced flexibility , weak core and back muscle.Sometimes a sudden onset can also happen with a disc injury which happens (the disc is a cushion like tissue situated between the vertebral bodies).Typically these symptoms should settle with rest for 2 to 3 days, with anti - inflammatory tablets and moist heat.Following the cessation of symptoms you must see a physical therapist to build in an exercise program for flexibility, core and back strength.However if the symptoms continue to progress, with aggravation during coughing/ sneezing,radiation or tingling/ numbness down the legs, weakness of lower limbs or loss of bowel/bladder control you need to see your GP or local health professional.