I have had lower back pain since march only on standing and walking?

I HAD A LUMBER SACRAL XRAY WHICH DID NOT SHOW ANYTHING, COLONOSCOPY TO RULE OUT BOWEL PROBLEMS AND BECAUSE I HAD ENDOMETRIOSIS THE DOCTOR KEPT PUTTING IT DOWN TO THAT. POST HYSTERECTOMY I AM STILL IN THE SAME BOAT. THE PAIN IS UNBEARABLE IN WORK AS I AM STANDING ALL DAY, AS SOON AS I SIT DOWN IT GOES AWAY. IT IS RUINING MY LIFE. THE ONLY MEDS THAT HELP ARE TRAMODOL WITH 2 PARACETAMOL BUT THEY ARE KEEPING ME AWAKE ALL NIGHT AND GIVING ME BAD THOUGHTS. PLEASE HELP. I am 37yrs old and Slim and fit. THE ONLY BACK TROUBLE IVE EVER HAD IS AN INFLAMMED SACRO ILLIAC JOINT 10YRS AGO.