Can't verify my suspicion without mod privileges, but since it seems you didn't know, removed posts don't show up in your rep history unless you mark the little box at the bottom of the page. Compare:
. . . vs. . . .
Since this checkbox also affects the total rep score displayed next to the date, it's quite a sneaky way to lose rep. Don't worry about it if this happens to you though; it's a matter of site maintenance, not anything you're doing wrong. E.g., if you successfully suggest an edit to a post that gets deleted anyway, you'll lose the 2 rep you gained for that edit. If on the same day another post that you downvoted gets removed, you'll get 1 reputation back from the downvote you cast. That could result in having a net -1 rep due to removed posts, which you'd have to check the box to see.