Game menu keeps popping up when I move

Both on single player worlds and multiplayer servers, whenever i move around using the mouse the game goes onto game menu. i played minecraft for about 2 hours at 8:30 til' 10:30 this morning, but now, at 4:45 in the afternoon, it has decided to do this.

REALLY HOPE YOU CAN HELP!!

A faithful minecraft player,

Imsy_Whimsy_Woo