actually I was under the impression that tolerance for benzos is actually rather slowly built when they are taken at constant doses. Regardless, several months will certainly build you a tolerance and Klonopin is twice as strong as Xanax. So you were taking equivalent of 4mg Xanax a day...except probably more b/c the long half life of the klonopin means it can build up slightly.
In short your few 2mgs Xanax's probably were just about your usual daily dose of klonopin...to which you are certainly tolerant...or at least used to.