Repeat after me: the BMI is crap. Repeat again.
And if you still have your doubts, read this: http://www.npr.org/templates/story/story.php?storyId=106268439
And here's a little personal example: When I was hyperthyroid, I lost weight. Unfortunately, it was all muscle. So in effect, I had a lower BMI, a lower weight and a higher fat to muscle ratio.
The best way to know if you've lost fat and not muscle is to try on your pants. Tighter = more fat. Looser = less fat. But don't forget: less fat/more muscle doesn't necessarily translate into lower poundage.