I could just imagine ProfX saying something like "do you have a Swiss ball too?" Anyway....
After reading this post, it sounds like you're still a beginner (which I still am too). The biggest help to you is analyzing what you're eating. Fats, carbs, protein, calories, the whole works.
The reason I said newbie was you said not eating after 6:30pm. That doesnt make sense man. If you're eating 5 - 6 meals a day, that doesn't pan out. There are plenty of people that even eat right before bed. So muscle mag told you not to eat after 6:30, not here.
My guess, and two cents, is that you aren't eating enough of what you should, starving yourself other times. Who cares if you're drinking diet soda if you aren't taking in enough water, protein, etc. I bet your body is starving from all the cardio you're doing and weight lifting. So if the body isn't getting enough nutrition, where does it go? To the muscle, and not the fat.
I could be wrong, and if I am, go ahead and correct me.