I have lumbar stenosis. when i sit i get a burning sensation in thighs and sometimes my knees that goes away when i stand could that be the stenosis?

YES: Characteristically, lumbar stenosis can be very position dependent, and flexion may make the symptoms transiently worsen, and extension may relieve. Since you have a structural condition, it is unlikely to easily respond to conservative treatments, and you may require more invasive approaches. Talk to an experienced back surgeon who could provide further advice.