I just lost 93 points in one day, with no downvotes, and it matches my /reputation calculation. So it seems like someone is deleting a lot of stuff, but they'd have to do a recalc for that to impact me. So did someone go an do a sitewide rep recalc?
EDIT: It's happened on other SE sites also.
EDIT: Looks like they decided to punish the users with site-wide recalcs across all sites, as per here.