Leg problem. Reason behind this thing?

I did not have any leg problem since birth. Everything was normal. One day in class 9, suddenly, my legs started to feel very heavy. It's not like a normal leg pain. The pain is because of heaviness. So, each leg started to feel like i am carrying 10 kg weight whenever i walk. I did not fall or no injuries as such. Since then i am facing difficulty. I am not able to walk straight or even stand straight. Leg feels heavy always especially knees. Also, my standing posture looks bad as if my knees don't touch like a normal person's legs would. There is little gap between my knees since then. I have heard about bow legs. But i don't think i am having that because everything was normal until class 9. I wanted to know can it be fixed anyhow? Can any exercise fix it? I am ready to work really hard for it. And what may be the reason behind this thing? Can i ever walk normally like i used to. I only wish that much to happen. Please do let me know.