Featured Images changed by themselves

I have Featured Images set to nearly every page and attachment on my photography site and realized today that many of them changed seemingly by themselves. Since there are no cobblers making shoes behind the scenes, I have to presume something I did made this happen, but I can’t for the life of me figure out what. With 376 Attachment pages and each having its own Featured Image set (so it appears as the background image to attachment pages), this is a really big deal, as I’m going to have to go through all 376 manually to re-set them.

The ones which were changed were changed in groups. So, for instance, 20 of 24 squirrel photos were changed to the same squirrel photo and 18 of 20 wildflower photos were changed to the same wildflower photo. And the Featured Images which “selected themselves” are images which I haven’t even looked at in weeks. It’s the weirdest thing!

It also seems like my categories are blowing out — categories I *know* I set aren’t set anymore. May or not be relative.

I have 33 active Plugins and 97 I’ve installed/tried/deactivated for one reason or another. Any one of them could be the culprit, and I have no clue how to reproduce the problem, which makes this seem insurmountable. I’m posting here with the hope that someone, somewhere has encountered this behavior in their WP site and will respond, in an effort help me step toward isolating the issue.

I’m using WordPress 4.4.2, a child theme of SKT Photo World Pro, and everything is set to NOT update automatically, until I get the site fully situated.

Thanks, so much, for any direction you can provide!