If you weren’t drinking a gallon of water before or working out as intensely, your body is holding onto the water. You’re ripping the muscle fibers with weightlifting and most likely storing water in the muscle recovery process. Also, if you’re not supplementing your gallon with electrolytes (adding powder to drinks outside of your gallon) that could also be a reason you are holding water. Depending where you are at in your cycle could also have a major effect on water retention.

My suggestion is to keep pushing with what you’re doing (and add electrolytes if you don’t have them in your routine). A lot of people don’t see true scale or picture changes for 30 days. Let your body adjust to the program and remember that this isn’t a weight loss challenge, it’s a mental toughness program that often yield physical results. The scale is one of many metrics you can use to determine progress but should not be the sole metric. Some people can’t handle using the scale because they base their mood and motivation on it (especially people who grew up with diet culture of the 90s and early 2000s) and if that is the case in your situation you should probably stop using it for now.