I just passed out.

So just now, I was sat on my bed and I got up to do something, and while I was stood up I started stretching. I got sort of dizzy and my heart felt strange. I heard a crash sound (I think it was me falling onto my dresser), and I woke up lying on the floor. I knew what had happened but it was sort of like i'd imagined the whole thing and it took me a minute to realise. This has never happened before, and I was really scared but no one was around. I always feel really shakey, like all the time but epsecially after I collapsed. I think i'm pretty healthy, I eat well and all that. I don't know why it happened, Is there something wrong with me? :-(
(I'm a thirteen year old girl by the way, I forgot to mention that)