This sounds somewhat unusual. Try turning off the system completely with the switch on the back. Turn it back on and then hold down the power button for about 5 seconds. This should hopefully get your PS3 and tv synced up again on the resolutions.
A search of the official sites Knowledge base turned up that the problem is likely caused by a mismatch in the output settings between the tv and the PS3.
Only other suggestion i can think of is to check the tv's settings- maybe it did not switch back to the settings required for the games (may have stayed in 1080i instead of automatically going back to 720p).