Garry's Mod Video Problem

Hey everyone!, ive recently re-installed garry's mod (if anyone knows what that it - its a fun "sanbox" game on steam)....anyway, after I re-installed it, and i start a map,I get these random flashes on my screen, when i turn a certain way, it will get brighter and brighter till its like a white out, its annoying so if anyone knew my problem, that would be AWESOME, and ive already tried updating my graphics card drivers and restarting my machine....any suggestions anyone?!?!?!?!?