Its on, as the car has sensed it is now riding differently! These are the old sensor types, not the 'fancy' ones that actually read air pressure.
Since you changed tires, its totally normal to have to reset the pressure system. The next time you for a drive, start the car and then hold down the (_!_) button until the light changes from red to orange. After that, just drive off. It will read the way the car drives now as normal, and in a few minutes the light will go off for good. (at least, until the next time the ride changes, whether due to air loss, a flat, or more new tires)
The system is operating as normal.
