What Causes The Left Hip Weak And Hurting When Walking?

I am 70, weigh 143 lbs. and 5ft. 5 1/2 in. and I am very active. I have a problem with my left hip being weak. My orthopedic surgeon did an x-ray and found nothing, but discover I had bursitis and sent me to physical therapy. Got rid of the bursitis and the hip still hurt when I walk up and down stairs. Went to another physical therapy and they decided it was the L4 and L5 of the disc that is causing the problem. Now I am doing exercise. So far I haven't seen any benefit. Forgot to mention to you that my lower back does not brother me.