But then, something doesn't fit in. According to FAQ up-votes of any kind can give me rep by multiples of 5 - but I lost 2 rep. How?
Here are the possibilities I can think of:-
You accepted an answer but the posts owner got deleted and so did his/her posts (which is rare, normally ownership is transferred to Community), rolling back your +2 rep;
You edited a post and the OP's account was deleted, rolling back your +2 rep (thanks to @nhahdth's comment;
A user who had upvoted 1 answer or 2 questions and downvoted 4 posts (i.e., questions or answers) had his account deleted (see this JSFiddle to calculate more combinations);