It could be muscle resulted spasm as you have ruled out other causes like disc etc with investigations..
I advice you to visit a Physiotherapist where he will train with exercises and stretches
Nite with medication and other therapies muscular issues never subsides it's only strengthening exercises will help but to see the effect of exercises it will take some time so confident and positive and don't give up
Next Steps
check your working posture and follow back are Technics like not bending forward not sitting at same place for very long time will certainly help if you practice regularly