Hi, if muscle spasm is happening just in 5 min of standing that means you are having bad posture ( sway back) or hyper lordosis. Hope you didn't had any injuries at your back before and no active inflammatory disease. Please rule out these red flags before getting treated from any Physiotherapist or doing any exercises.
Next Steps
if you do not have any red flag signs then in acute phase do very simple exercises and moist heat therapy with posture correction ( mechanical low back pain). Don't go for aggressive exercises otherwise it will aggravated your symptoms
Health Tips
Please clear red flags signs ( acute injury, inflammatory causes etc.), don't bow , do not sit on floor with crossed legs ,no heavy weight lifting and if you are a bike driver then please keep bike tier properly and adequately inflated.