It's most likely the salt, don't worry. Do you weigh every day? If you do, you aren't going to see a steady down drop every day. Some days I'm way higher than I should be, for whatever reason. I just shrug it off, go exercise, eat right, then hop on the next day, and more often than not I'm very pleased. Also, as weird as it is, the last time I had a HUGE gain that I thought couldn't possibly be from water, I had a huge WOOSH the next morning. It was weird.
Also when I know I've eaten a bit too much salt I just try to drink as much water as possible to wash as much out of my system as I can before weigh-in.
Don't get discouraged, it's just one day.

Just pick yourself up, dust yourself off, and give it your best today. You might be surprised the next time you weigh. <3