What Causes Shaking In Arms And Legs ?

im 13. it aint no sad story or what ever but i got some stuff that went on a few months ago and it dont seem to stop so pretty much i always got somethin thats happenin. and ive been like just randomly shakin in my arms stomach and legs out of no where. and it last from like 30 - 40 seconds like im shakin right now. its not so bad i cant function but its REALLY annoying. and i cant focus. it happens usually like 4 or 5 times a day. i usually shake when im excited or nervous or upset. when somethings happenin. so any clue what it is.? anything will help?