The compression of the sciatic nerve should be around the right side that's the reason you have pain in the left side. That's the reason you cannot sit or stand and it is exactly around the L5 vertebrae where the sciatic nerve passes by. It can also be due to the strain on the left leg, try wearing MCR chappals which would help you to get relieved from pain
If you have low back pain
you can undergo intermittent pelvic traction
where in your half of your body weight will be added in the electronic traction so that that would help you to reduce the pain
, along with Interferential therapy
stimulations which would help you to get relieved from the radiating pain. Apply ice for the pain to get subsided immediately.