Fluctuations like you see are just salt, or possibly alcohol, causing water retention. There's no way anyone who isn't in a full-out binge would overeat by enough in two days to add 2.5 pounds of fat (that's about 10,000 calories over maintenance). That doesn't mean it's necessarily a good thing to "let go" as you describe -- it may or may not be depending on exactly what you're doing and what your personality is and so on -- but it's a separate issue.
As you've seen, water gain based on something unusual in your diet will generally go away after a few days.
There's also hormonal/monthly water gain. Personally, I have seen shifts of up to three pounds overnight because of this. For me it can take anywhere from a few days to a couple of weeks to reverse itself and start to go down again (or up, now that I'm more or less maintaining).
I can also see fluctuations of up to about two pounds just within the course of a single day.
For me, knowledge is power. I would rather know that a given weight is off the overall trend and therefore probably meaningless, than weigh only every 7 days or less and have to guess. So I weigh on average daily.
Height 5'8 1/2"
5K 4/21/11: 31:55
|58 Maintenance Weeks