It could be any of a number of things, but I'm not sure 2 qts. of coolant would be enough if you were low. One of the first things I'd check is to make sure you're up to the right mark on the expansion tank. If on braking or acceleration the fluid moves to expose the sensor in the tank, it can trigger momentarily the check engine warning. These tanks are also prone to deterioration, so check its integrity, and also that the sensor probe is clean and otherwise uncompromised.
Joel