It's looks that on long standing usually our muscles get fatigue and also on long standing there is pooling of venous blood and lactic acid accumulation can cause pain , another possibility is to rule out venous infuffiency i.e. Varicose veins and lastly we need to also rule out L4L5 disc prolapse, you need to show to an orthopaedic surgeon and also try to change posture every half an hour e.g. Sit down for a minute after standing for half an hour or so . Drink plenty of electoral powder and take tab Carnisure 500 mgs twice daily and tab Evion 400 mgs once daily and tab neurobion plus twice daily for 6 weeks . If you don't get alright after 2 weeks of medication then please do visit orthopaedic surgeon or if you feel uneasy with any of the above medicines then please stop them and show to an doctor