Vauxhall Astra brakes problem

Hi guys, car has developed a problem. When I press on the brakes anything over 30, the car feels so unstable like it is moving around and feels like its needing more to stop it. I've taken the wheels off and cant see any damage or leaks. The tyres are good and up to pressure. The calipers were replaced about year n half ago with discs n pads. Only started on Monday on way home. Had to stop quick and car felt very loose and felt like it was not going to slow like there was no force or pressure but the pedel felt ok. Very hard to describe but defo something wrong. Help......