TrimT muscle doesn't technically weigh more than fat, it's just more dense, so takes up less volume, so lots of people think muscle weighs more than fat. I've said it many times as it's a simple way of explaining it and I always get told off for it! I think I've seen KD and Laura Croft say about this as well. The thing is, when you're on CD you're running on a calorie deficit so you won't be gaining or building muscle, there's just not enough calories and protein in your diet for it. BUT you will be toning muscle which is more what you want at the moment anyway. So you won't be gaining loads of muscle weight as you can't build it up, but after exercise you can retain some water which may give you a gain or STS on the scales, or in your case a small loss, but that isn't necessarily correct and could be a bit misleading as you've got a bit of water weight to go. I don't know just how much you're exercising, but if you find that your losses are often effected, try cutting it down a bit. With the cardio putting in a mix of resistance training (weights) might help you too. With resistance training though, don't feel you have to go for big heavy weights, you don't! You're better off using lighter weights and doing more repetitions. I don't know what equipment you've got at your gym but I'm sure if you ask they'll point out which machines tone which areas, there's sometimes a book in the gym hall too. You may already know all this, so if you did I apologise if it came across as patronising or anything, obviously isn't my intention! Just never know what knowledge people have.
Oh and torso twists are pretty good for toning that area too!