The odds are slim that there is not a problem bigger than the grout. I am betting that the floor is flexing, and has loosened the tiles. You can determine this by tapping on the tiles, if they sound hollow they are loose. If this is the case you will have to take up the tiles and fix the structure problem. If this is not the case you can regrout the tile, you do this by getting a grout saw, sawing the grout, shop vac the floor, regrout the tile.