If it didn't leak before the fluid change, but leaks afterward, it's possible the dealer did a poor job cleaning up the mess made when the tranny fluid was changed. Crawl under the vehicle and clean it up, then drive it a bit and look for look for leaks.
As far as the tranny acting acting up, I can only speculate the tranny fluid was flushed moving debris into places it doesn't belong, causing new problems. That's why I recommend a fluid change, but never a flush. And if there is a tranny filter, hopefully it was replaced or cleaned, but many shops are lame about this too.