Over a matter of three months I lost just a few pounds, maybe 5, and had to get a pant size above what I've been wearing for years! I don't understand, thought maybe my scale was messed up, but its not. I don't feel that I've changed my eating habits, and I'm not trying to lose weight. I don't drink anything but water, milk, and juice. I can tell a difference when I look at my stomach (I have a pretty small frame), it looks like I've gained a little weight. And its not exactly comfortable to me when I'm sitting up. What happened!?
Hmmm...that does seem odd. The only thing I can think of that would add bulk or muscle to your waistline causing you to go up a size in pants...would be if you were working your abdominals with a lot of weights. I've heard that using weights or weighted abdominal machines will result in a "thicker" waistline. I always see the guys at my gym using heavy (like 45lb plates) on the ab machine. I use no more than a 5lb plate (not that I could even remotely do it with a 45lb plate on, mind you!).
Are you drinking skim milk? Juices are pretty high in calories, so maybe cutback on those. Sorry...that's all I can think of. Hope you can figure this out.
When you first start weight training and if you have a lot of fat and don't change your nutrition, it is quite possible that you can build a layer of fat under your muscle, bulking up in the short run.
however, if you have not done/changed anything about your lifestyle, i suspect you are losing lean tissue and replacing it with fat. women start losing lean tissue naturally at 25, and lose it faster with each subsequent year. muscle weighs more than fat in terms of volume. if you have 5 lbs of lean muscle tissue and don't maintain it trough a consumption of adequate levels of protein and weight-bearing exercises, it can very easily be lost and replaced with 5 lbs of fat which will make you 'bigger' (as 5 lbs of muscle is physically smaller than 5 lbs of fat). maybe, who knows, you've lost 10lbs of lean muscle, and gained 5 of fat, slightly changing your body shape.
The body can do strange things! It's possible that you lost weight in some areas, but maybe other areas got larger (but still resulting in overall weight loss), causing a larger size to be needed. Sometimes I seem a bit bigger at first when I exercise more, THEN get smaller. My muscles just get bigger before I start losing fat. Also, are you sure maybe the clothing didn't run small? I find that so many brands sizes change each season. I'll wear one size at one place, another size elsewhere, then the brand I always get runs different out of nowhere!