I played for 30 mins and i've reached 3rd level in Multiplayer, after i restarted game, re-entered multi cd-key ( 2nd time) i am Level 1 Again. Why that? How can i get back to my 3rd level ?

Because you played on a unranked server. Play on a ranked and your stats get saved.

yeah, not always, infact not all that often. I only play on ranked servers and I loose exp and unlocks all the time...

They really did a crap job on this side of things.

1+