As The Builder says, sometimes a bulge is due to the failure of the screed laid on top of the floor. My next door neighbour had a similar problem - she was convinced that the roots from a tree outside had come into her house and had caused the floor to rise up in places. When I lifted the carpet, I discovered that it was just that the screed had cracked and disintegrated in areas of heavy wear and tear.
If you lift your carpets, can you take a picture of the *naked* floor (put it on something like photobucket) and give us a link to it.