Weighing yourself once a week provides "noisy" numbers. Somewhere within the noise is the change in your fat and muscle volume. Also measured on the scale: the amount of food in your gut, and water weight. Each of these things account for AT LEAST a few pounds either way, so it's totally hopeless to try to measure a body composition change as small as 1lb. Even our air exchange accounts for a daily weight change of about 1lb, although this too can't be measured unless you can exclude all other factors (eating, drinking, eliminating, sweating).
Take the weekly numbers and smooth them out or compute running averages. After several more weeks, the numbers become more meaningful. But, really, paying attention to temporary 1-5lb changes on the scale is utterly pointless.