Have pain in left groin &hip aggravated by sitting. have spinal stenosis severe in l4'l5. started 20 days ago after long car ride. can back cause?

Yes: Yes sitting long periods of time can inflame a lumbar region that as severe spinal stenosis. The pain in your hip and groin can be from aggravated nerve roots. You can try McKenzie Back exercises. If it doesn't resolve, then seek medical evaluation. Back pain that causes numbness around rectum, foot immobility or bladder incontinence needs immediate attention.