After doing my first back-load i weighed in about 5 pounds heavier.I doubt i gained 5 pounds in a single sitting...Do you think i'm just holding onto more water weight because of the carb-up? Or do you think its actually mass (fat/muscle)?
This happens to me as well. You should drop some of that every ULC day you do. If you're backloading more often than not, then you may have ingested too many carbs, but just keep an eye on it for a little while and track your progress.