Playing against underestimated player damages my rank at KGS
Your first question is largely answered by my answer to your other post. You lost a game to another player, whose rank was maybe the same as yours, maybe not, and that has to decrease your rating a little bit - in this case it was enough to cause you to drop (maybe you were close to the boundary between ranks, who knows). 

And it's all based on current rank, not past rank. The system can't pretend they're 6k just because they used to be. (Although it could be a sign they're trying to cheat, and they might want to boot the user - but in terms of the rating system, there's not really any alternative to using the current rank.) Using the current rank seems pretty fair; imagine a different case, where you won against someone rated 6k who used to be 13k. You'd be pretty mad if it pretended they were 13k, right?



The answer to your second question is yes, your rank will be updated if the system realizes that the other player was stronger. This is mentioned in the FAQ on their site:


  When are the rank graphs updated?
  
  Ranks are recalculated frequently, but the graphs change only once a day at 0:00 GMT. 
  
  Why does my rank graph change when I haven't even played any games in awhile?
  
  Two reasons:
  
  The rank of one or more of your past opponents changed.
  
  ...


So bottom line, don't worry about it. Presumably they'll turn out to be good and your rating loss from this game will decrease. And if they forgot everything in that 3-month break and really aren't as good anymore, well, you lost to someone close to your skill level, and the rating system gave you what you deserved.