Needs the guys help/advice...
I don't run but cycle and significant weight changes occur frequently for me as well. The cause is hydration or lack thereof. After a hard workout where I sweat a lot, I can lose upwards of 5 pounds which is regained over the next couple of days. On the other hand, if I "carb load" prior to or during an endurance event, I can gain 5 pounds which is lost over the next couple of days depending on how much I drink or eat during the event. For every carb gram you store, you also store four grams of water weight. Although water doesn't have calories, a gram is a gram. An ideal situation for your body (and performance) is if you weigh pretty much the same prior to and after an endurance event or workout. This indicates you properly hydrated during the event. Weigh less....you didn't drink enough and you risk "bonking". Weigh more.....you drank or ate too much and your performance will be sluggish. Boner.