It depends on where you are losing the weight. You may be losing a lot of it in one area. That or you are buying your clothes in different brands and none of them are measuring up to each other or even accurately. I make sure my brand is accurate before I buy and that it is always the same brand. I lose 1 size every 10 pounds (I know this from experience in my past when I was anorexic and losing weight and it still seems to be accurate).
Also what others have said you may have stretched your current clothes. I have clothes I've worn since 6th grade (Age 12..... I'm 17) And they are 13.... I am no 13.... They used to even be a little tight when I was 13. I totally stretched them out.
To make sure your clothes are accurate measure your hips and waist. That's what I did. My new jeans are indeed accurate. And then I used this...
http://www.garmentofpraisecoga.com/i...U.S.Misses.pdf to match up my measurements to the right size.