My rbc is 3.97*10^12/l (ref 4.5-5.5) mcv is 112fl (ref 83-101) mch 34.9pg (ref 27-32) mchc 31.1 (ref 31.5-34.5) lymphocytes 41.7% (ref 20-40). i am getting back pain and leg pain after walking for 5min. is something wrong?

There looks to be some abnormality for you at 18 years to have back and leg pain after walking for 5 minutes. You saw a medical practitioner is what you have done but you did not put enough information here. The blood test also shows abnormal red blood cells, I would think you should follow with the person that you saw to help you manage what is going on.

All laboratory results need to be interpreted in the clinical context and the doctor who ordered the tests is usually in the best position to do that. Talk to the doctor who ordered the tests. Having said that, you may have vitamin B12 and Folic acid deficiency. Take both. You may need B12 injection. Repeat the test in 4-6 weeks. Wish you good health!