I did some weight lifting. after that got some pain in back and knee. done x-ray test and got loss of normal lumbar curvature at l1-l2 and l4-l5. now?

Rest, rehab: Your symptoms are likely normal soreness from an increase in exercise. Though loss of lumbar lordisis often means nothing, it can indicate that your sore low back muscles are tense. Take it easy for a few days (ie. no lifting, continue regular activities). If symptoms persist you could consider physician re-evaluation and possible initiation of a thrapy program