For a few months i've been feeling like the vertebrae in my lower back are moving. today i lifted my backpack & my lower back painfully locked up. (?)

Back issues: Sounds like you may have some a herneation or some degenerative disease. You should see your doctor for exam and diagnosis

Spine: You have spinal and paraspinal degenerative disease;seek help with analgesics,anti inflammatories muscle relaxants;longer term adopt good postural habits and seek professional help from physiotherapist and your physician;correct any nutritional deficiencies including Vit D