Yes serious concern.
Loss of lumbar lordosis indicates spasm and listhesis reference to anterior or posterior displacement of vertebral body as compared to the one below...
You might be having a Acute PIVD secendory to listhesis.
Next Steps
Condition need evaluation with MRI.
Try:
Absolute bed rest.
Hot water compression.
Medications like Myoril, Enzomac, Neurokind LC will help.
Health Tips
Do not do spine extension exercises, it may exaggerate the listhesis part.
Spasm will relieve by 1-2 weeks , if you take complete bed rest.