I was disappointed to discover that my rep. had fallen from 11 down to 5. I'd appreciate if someone could help me out, thanks :)
Deleting posts which have a negative score and which have attracted a penalty thereby should restore the reputation lost. It's a little more complicated than that, because all votes are reversed.
An answer which has a single downvote (score −1) will have incurred a penalty of 2 points. Deleting that answer will restore those points. It also restores the −1 penalty incurred by the downvoter.
An answer which has a net score of −1 reached by an upvote and two downvotes will have gained +10 for the upvote and −4 for the two downvotes. Removing that answer will restore the rep lost from the downvotes but also the rep gained from the upvote. That will result in a net loss of six points, the same as the net gain it had from votes. Again, the downvoters' penalty is also restored.
A similar mechanism operates for questions, albeit with different numbers (an upvote is worth +5, a downvote −2, with no penalty for the downvoter).
You can see your reputation moves on your profile page, and the public view shows the movement resulting from posts which still exist (1 + 10 − 2 − 2 −2 = 5). You may be able to see more detail there than other users. Moderators have access to some slightly more detailed data than is generally available — which, just to be clear, does not include the identities of voters. I don't propose to go into that detail in a public answer.