You added 2 qts of oil? Was it that low, or did you just add the oil without first checking the level on the dipstick?

First, the pressure sensor might be bad. Second, the oil pump might be bad. But I've had almost about a million miles of redblock ownership over several cars and never had a problem with either of these, so I'd be inclined (personal opinion) to consider the oil itself as most likely. Consider:

1) if you overfilled the crankcase, you may now have oil foaming (when the crankshaft whips up the oil into a foam), which pumps the foam, instead of liquid oil, and results in very low pressure (and little actual lubrication) which might make the idiot light flicker. Adhere to the maximum mark on your dipstick, and drain out what is excessive pronto!

2) if you needed to add 2 quarts because you were that low, maybe you've neglected your oil changes, too? The oil may have deteriorated, which causes thickening eventually -- and cold weather just makes matters far worse. Sure, thicker oil usually results in higher pressures once it starts to circulate, but initially on a cold startup, it takes a while to pump up that thicker oil and you may have too low a pressure (causing the flickering) until the thicker oils starts to circulate properly.