I have a mystery leak in the bathroom?

Hi - has anyone get any ideas about this? I have a leak in my living room ceiling, which is below my bathroom. I have checked all the pipes round the bath and toilet and can't find any wet patches. However the problem seems to occur when I have user my shower - which is just one of those push-on jobbies fixed to the bath taps. It seems like the problem occurs when I am using the shower mixer, e.g. when I'm washing my hair (ie not in the fixed position). Although I get enough water pressure for what I'm doing I'm wondering if the water can be backing up somewhere and leaking further back (ie under the bathroom floor boards). I really don't want to have to get a plumber in if I can help it because my bathroom is so small that I know he will have to take out the bath,and it was fitted so stupidly that there's no way of doing this without damaging it, meaning I will have to add the cost of a new bath to any other work! Any ideas?