Biologically, we are not designed to walk upright. Our skeletal structure is still more suited to support from our knuckles dragging on the floor, supporting our weight. That is why humans suffer so much with back problems.
Your condition may be caused by slightly displaced vertebrae which will cause undue weight transference on one side of your body, which will make you limp. You may be totally unaware of this reflex response, because you feel no pain, and your brain does not register the correction impulses being directed to your legs to take the train.
As advised, have a chat with your GP who may be able to refer you for some physio - the condition will only get worse if left untreated, and could store up problems for later life.