You have 2 cards in your hand, leaving 50 unseen cards. There are (50 choose 3)=19600 different flops that can come up. (2 choose 2) * (48 choose 1)=48 of these give you quads. Probability of flopping quads is 48/19600.
Would you believe that once I flopped quads with a pocket pair 4 straight times? Well, you shouldn't.