Suddenly my legs weight got increased.

Hi, Initially I am suffering with back pain, I have gone with some medicines as prescribed by doctor, for time being my problem got resolved. After 2 months again started back pain and in a weekThere is sudden weight increased in my legs( below knee untill feet) and observed some black spots on leg. If I am touching those positions severe pain observed. Again visited doctor, he suggested belt for back support and he said some infection is there in legs. He has has given medicines for 5 days. As of now three days completed, but I haven't observed any change in legs. Still I am having the same pain in both back and legs. In X ray there is no abnormalities observed. Can any one suggest any medicine for this.