The cooling system is pressurized and when you took the cap loose you released the pressure and the extra coolant that was allowed to fit in there with that pressure. Look at your radiator cap, on top, it will tell you how much pressure it is holding in the radiator (if it is working right anyway).
The last time you ran it, the pressure was built up and because you have a good , non-leaking cooling system it held the pressure even after you shut it off. It means everything is working right.