If you have any low voltage lights upstairs then it could be a faulty transformer. Don't remove the lamps just make sure the switch is in the off position. The bottom of the switch should be sticking out if it's off (providing the switch is'nt upside down) If the breaker then resets and it trips when you put the low voltage lights on then it's either a faulty transformer or a dodgy lampholder, these break after a while cos they get fookin hot !!!

It could also be damp / condensation in your bathroom fitting