Water issue

I have noticed when I empty my bath or bathroom sink that I get water coming in through the back window in my backroom. I have so far managed to prevent this by emptying the bath water either down the toilet or scooping it out and getting rid of it via my kitchen sink. I have however noticed that on my kitchen wall (it is bare plaster at the moment) the wall looks wet (no running water down it) but wet and it is gradually spreading. Is this a problem with my guttering or something more serious. Who do I call a plumber or other tradesperson.