vpopovic wrote:I was promoted to 8k the day before. ... Now, I'm 9k again

Many people don't understand the difference between
level and a particular
rating.
That's why we see questions like this one all the time.
We don't know what's your true
level.
If your true level is 9k KGS, it means that from now on,
if you play approx. 1,000 games on KGS against other solid KGS 9k people,
you'll win approx. 500 games and lose approx. 500 games.
If your true level is better than 9k KGS, then you'll win more than 500 games.
If your true level is worse than 9k KGS, then you'll lose more than 500 games.
Just because one day you make 8k KGS does not necessarily mean that is your true level.
Your level is absolute, intrinsic to yourself;
your rating is relative, external, and may change from one server to another.
If you want to find a KGS rating that accurately represents your level,
then play many serious games on KGS. The more, the better.
After 100, 200, 500, or 1,000 serious games, your KGS rating will much more
accurately represent your true level.
vpopovic wrote:I haven't played for only one (1) day and I lost quarter of a stone. I don't understand.
You want to improve your level, not your rating(s). Forget about your rating(s).
Your level is the same from yesterday to today -- it has not changed (much).
Your rating(s) will follow your level; not the other way.