Just noticed that when I switch on my bedroom light the lights in the kitchen and dining room don't work. When the bedroom is off they work again.
There are 3 live wires to the bedroom switch and 2 each to the other two.
The switch can have 2 or 3 woed depending on how things are wired, but turning on one ought not pinch the electric from others :-) It sound like some fool has switched electricity from one route to another when you use the bedromm lightswitch.
If your house is relatively new, or recently rewired, the only wires you would expect to see in the switch would be the "switch wire" coming from and going back to the light fitting. The live feed wire normally goes from light fitting to light fitting, one for each storey of your house.