I *think* -could be wrong here- takes more than a couple days of consuming less calories than you should for your metabolism to all of sudden start storing fat for starvation, unless you are REALLY not eating.
It probably has to to do more with retaining water, especially if you've been too busy to make sure you drink the recommended amount. Also whenever I eat pasta or anything really carb heavy the next day it looks like I've gained weight, but it goes away after a day or two.
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