Why does my back ache?!

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If U are carrying a laptop or other heavy objects, this can cause you back to be out of alignment. Use a backpack with a built-in roller with handle, or put the heavy object down. Don't put it between your legs, this will cause your stance to be too wide.
I use to walk down to the earlier stops on the bus to get a seat to sit down.I don't know if that is possible in your situation. Maybe take a later train that U can sit down on?
It helps when standing to be able to put one foot up a little bit, althought
Building up your muscles in your back will also help, as will stretching. Yoga will help in building up your muscles as will swimming.

Do some exercise for the spinal chord and back bone. chest forward, stomach in with contents, head backwards , Afterwards do some stretching bending, turning left and right. You should not have pain during work out and it will be a relief. No pillows during bedtime.

you could try pilates or yoga to start, and then get on something like the trx suspension trainer to strengthen it. My brother had to use it for fitness training to get strong

http://www.escapefitness.com/trx-pro-suspension-kit.html

you should go to doctors immediately.