I am having constant pain in my left leg. the pain starts from the lower left back and left pelvic region and radiates down to the left leg. though so?

Needs evaluation: An evaluation would need to be done. Do you know of any unusual activity or other possible cause? The muscles and nerves in the lower back/hip region are many and complicated. Misuse can be subtle - sitting oddly or too much, even stuff in back jeans pocket can press on them and cause this sort of thing. A physical therapist or massage therapist can Dx and treat. Meanwhile, try heat/rest/gentle