Log your food. All of it. Know exactly how many calories you're eating every day.
Know how many calories you're burning every day. You can use an online calculator - webmd has a pretty good one.
Then look at the calorie deficit you've been creating. When you have a large calorie deficit over a period of time, your body thinks it's not getting enough energy and hangs onto everything that it can to protect itself. If this is the case, take a couple of days and eat a little more - closer to what you're burning. You don't need to go nutty, just create less of a calorie deficit and see what happens. Or maybe you'll find that you haven't been creating much of a deficit, in which case you should work on that!
Man who say it cannot be done should not interrupt man doing it - Chinese Proverb
"Eat food, not too much, mostly plants." Michael Pollan