i lost 6 level on steam

i had "only" 300 games so It's not only that, i sent an support request for know why, if they gimme an aswer, i'll post ya here.

[Thought I'd expound upon your comment a bit with some numbers. ^.^]

Well, for a user with a small library it's possible*, but not at the OP's level and library size, no.
If I recall the formula for it correctly, it should actually be 1110 xp per level, at the level range indicated.
Since games over 1000 give exactly 1XP each, the total drop should have been around a quarter-level.

(*Formula for first thousand games.)

Definitely seems like a bug, from the information currently available.