Lneal you also have to take into consideration that muscle weighs more than fat...
Also, what peachy said is true to a point... yes your body does go into a starvation period, and a cheat day is good... but also look at it this way... Your body will go into starvation mode the first week or so when you start a diet... Thats why week two of a diet is the hardest and you barely lose a pound. But after that your body jsut says... F*** it we'll do it your way... and then you start to lose, even with the cheat day (if done right... just cause you can cheat that day, doesn't mean to go over board)
So you may be gaining muscle!