Right leg and hand are tired and fatigued?

Hi, I'm 15 years old and I'm experiencing weird symptoms. My right leg, and right hand/arm feel heavy, tired, and fatigued. Writing in school is affected a bit, and thats really it. When I'm not thinking about it the symptoms are usually mild. I don't have spasms nor am I in pain; there just "lazy" if you may. I've had these symptoms since a while ago (can't remember exactly when), but they dont look like they're progressing. My left leg and arm have no symptoms what so ever. Could this be something serious?? ALS maybe? Im very scared.