If it wont stop, even if you dont add any new coolant, something is definately wrong... .
I once had it (a lot of overflow after real short trips). It started real sudden, and was really bad.
We tested the coolant for Co2, and it showed that there was Co2 in it. So that meant there is a leak!
After all we tore the engine apart and replaced all major/upper gaskets.
Since then everything works fine again, well kind of. (knocking on wood).
In my case it must have been some faulty/broken engine gasket... . /forums/images/icons/frown.gif
NOT FUN.
Hope its not as serious with your car.