Yeah, don't worry about the weight change. It's a new scale...in all the time I've been on this forum I can't recall a single time someone tried a new scale and it read exactly the same as the old one - and that's a lot of posts.
You're definitely better off than some people who saw a 10lb gain!
Fast food and restaurants in general are notorious for dumping sodium in their food. If you're not used to that level, it will cause a major spike in weight. Try to flush it out by drinking lots of water today. Hopefully you'll see your old numbers again!