we just had thanksgiving! that happened to me too. there are many reasons why a person's weight or distribution of their weight might occur temporary or permanently. for instance a teaspoonful of salt can hold a quart of water in your body weighing about 2 pounds or so. you didn't mention if you were male or female but women who are having regular menses often experience monthly changes in their pants when they are holding water as a result of hormonal influence. putting on a few pounds of fat also occurs with people on a regular basis. i have a closet full of pants that are just too tight now. my theory was that they spontaneously shrink over time but no one including my bathroom scale believes it. on the non-body side clothes can shrink when you wash them so if this is happening buy bigger pants. pants that expand a bit. and avoid those pants that take grunting and a shoe horn trying to put them on.
i could fit into my pants one week but not the next?