Temperature change. My daughter goes sometimes when taking her nappy off. Not because she's a little monkey pants (which she can be!), but because of the change in temp.
As for showers, it's the sound of running water that makes you want to go. Not sure why. Tell me about your mother ;o)
gingerflaps - I get the same thing!! Its wierd isn't it. I also get it when I go in a swimming pool which is worse (unless its a cold pool)! But as long as I have just been to the loo before I get in it is all fine - just a sensation I think. I don't get it in the shower. I think it is a temperature thing though - sometimes I get it after getting out of the bath or swimming pool too.
Makes me feel like an old woman sometimes! (I'm only 24!)