Beer becoming more carbonated over time

I just opened up a bottle of Stone Vanilla Smoked Porter which I've had in a cool(ish) dry space away from any light for about 18 months. I haven't had another Stone Vanilla Smoked Porter to compare it to recently but this one has taken on the mouth feel of champagne. Super carbonated, but still tastes like the beer I remember it as. Is this common if I didn't cellar it in optimal conditions or do I perhaps just have a bad bottle?

Thanks for any input.