Squeaky floorboards under a tiled bathroom floor?!

[ModernPerfect]

I have a 1933 house and had a new bathroom put in several years ago.
It's been fine up until a while ago when I noticed a creaking sound when standing in the bath. Lately there's a couple areas on the tiled floor where it sounds like you're standing on a squeaky floor board and I can feel slight movement of the tile underfoot.

I'm concerned some moisture has started affecting the floor. Is this a normal problem to have after a period of time? How concerned should I be?

 

[blup]

The creaky bath might be the bath and bath legs direct to the floor, often tiles don't extend all the way under. The tiles might crack eventually if the movement in the sub floor continues especially as the tile is moving. You will have to keep it under review.