3 days before today I went to my school and had decent amount of physical activity. Later that evening I went to the park and ran for around 20 mins. Also, I am underweight. Since then there is constant fatigue in my legs. My body is not willing to just stand up on my legs. As soon as I stand up, my body says to just find a chair and sit down on it. I am not willing to play any sports even though I like them very much. Even when I am lying I am more relaxed when my legs are straight rather than when they are folded. This is the first time somehting like this has happened to me. I am really tensed beacuse of it. Why is this happening ? What is it's cure ?