I copied this from a thread on another site I used to use, it might explain your dilemma:
WHY THE SCALES CAN LIE
Why we work so hard, but nothing seems to change...
A biologist at Berkeley shared something very revealing on the low-carb BBS system about 4 years ago that helps us all through the erratic weight fluctuations you invariably encounter: Fat cells are resilient, stubborn little creatures that do not want to give up their actual cell volume. Over a period of weeks, maybe months of "proper dieting", each of your fat cells may have actually lost a good percentage of the actual fat contained in those cells. But the fat cells themselves, stubborn little guys, replace that lost fat with water to retain their size. That is, instead of shrinking to match the reduced amount of fat in the cell, they stay the same size! Result - you weigh the same, look the same, maybe even gained some scale weight, even though you have actually lost some serious fat.
The good news is that this water replacement is temporary. It's a defensive measure to keep your body from changing too rapidly. It allows the fat cell to counter the rapid change in cell composition, allowing for a slow, gradual reduction in cell size. The problem is, most people are frustrated with their apparent lack of success, assume they have lost nothing, and stop dieting.
However, if you give those fat cells some time, like 4-6 months, and ignore the scale weight fluctuations, your real weight/shape will slowly begin to show. The moral of the story - BE PATIENT! Have faith! YOUR APPEARANCE AND SCALE WEIGHT MAY FLUCTUATE, EVEN BACK-TRACK, BUT YOUR RESOLVE, COMMITMENT AND FAITH SHOULD NOT!
Does that help? Get out your tape measure, you might be surprised.