Pan in back of leg and tigh and numb foot , what can it be? i work all day sitting down?

Should get MRI of lumbar spine. Symptoms could be caused by a number of things

Sounds consistent with sciatica and maybe pinched nerve in lower back or piriformis syndrome. Start with lumbosacral MRI

It could be compression of a nerve around the lower back or hip. Start with physical therapy and if no improvement or worsening have your primary care order images of low back and hip