I have lower back pain mostly on right side. When i stand after sitting position i could not regain my normal posture and i could not able to sit straight when i sit on a chair after standing. It look like the nerves are pulling or straining when i stretch my right leg. Feel like a cramp and after some time of sitting posture i feel it normal. But i could not stretch my leg straight while sitting on a chair.

Have you made an appointment with a doctor yet? If not, it really is time you do. No one can accurately diagnose your problem without doing a physical examination. Good luck! I do hope it is nothing serious and simply a strained back muscle.

We can only make suggestions, and you should see a doctor and a chiropractor.
It would help to know your gender.
You can't see nerves, so if you mean visually see, you are seeing muscles or tendons that have tightened. If that is the case, I would go directly to a good chiropractor rather than an MD.
An MD could sign you up for physical therapy. There are many good exercises for the lower back, and tips on how to sit, stand, keep warm (keeping the legs warm can help relieve lower back pain in cold weather!), go swimming in a warm pool/sit in a whirlpool, and of course - never wear your wallet in a back pocket!
I know several people (my dad included) who sit at their computer at an angle with one leg off to one side as though they are just going to be there for a second. SIT FACING FORWARD, both feet flat on the floor, don't hunch, butt at the back of the chair, arms resting on a surface that is the correct height (the height your arm would be if bent in the air)

There could be more serious conditions in any and all complaints of pain, if the typical approaches to nerve and muscle therapies fail.