I just checked your profile and you must be one of the most constant Monster players, having recently passed the 30,000 mark (you are on 30,780 as of today).
Maybe there is a problem with the game database beyond the 30k boundary - only Terry can check that. I could imagine that when it was originally built, it was never expanded beyond 30k and a bit of safety margin (enough questions for five and a half years) and that you have reached the edge of the known universe
By the way, did you get points for the questions you did not play (i.e. a score out of 15 with the unplayed ones marked correct)?