What Causes Burning Sensation During Urination Along With Inflammation Below The Knee?

There is burning sensation while passing urine during the day. Also lower legs below knee also have that burning sensation. This happens during the day when I lye down for rest but during the night this does not appear. I am over 75 years and there is nothing wrong/no infection as I have got urine routine and culture done and so also all types of blood test without any thing wrong. I take simple veg . food , milk, curd and drink a lot of water. This problem is for over three months. I am on medication of a Sr. Doctor who is urologist. I met other two urologist too but there is no relief in my problem. What should I do.