Oh. Turns out they put the battery in the scale backwards*. Huh. Well. The bad news is, I have gained a few pounds (7) but the good news is, it isn't 20. This I can deal with. I'm still not sure what to attribute it to, although on second thought there was an awful lot of ice cream with my niece G. this weekend. (I swear, she should have been my child: she hates getting up in the morning, she trashes her room, and she lives on ice cream. I am so proud.)
*it's one of those disk-shaped LCD batteries, in case you were wondering how a 9 volt got put in backwards and I didn't immediately notice.