Movement input is reversed

Sometimes my input (forward/back and left/right) will get reversed, similar to when you get hit by CS gas. This seems to happen at the beginning of rounds and I'm not sure what causes it -- usually it happens when I first get in a server but sometimes it will happen after a round or two. Also, controls go back to normal if I use the Bloon cam, but only while flying the Bloon. Ladder controls are reversed too (trying to go up goes down).