Just to be clear, by "rest days" you mean the days you're training for probably 45 minutes or so first doing high intensity weight training (complexes) and then 20 minutes of cardio.
My thoughts are that you're severely underestimating the intensity of these cardio sessions and neglecting their impact on overall recovery, especially if you're trying to focus on building a specific bodypart in your main workouts.
Couple options to consider: First, like Moog said, make sure you're having some carbs around your workouts, at least before and during. Most coaches will agree that peri-workout nutrition is the last place they remove carbs/calories, even when dieting.
Or, if you want a better carb cycling approach, stick to the basic high, medium, and low days, but really understand what those days are. If you're prioritizing chest, then those should be the high days. The "rest" days when you're doing complexes and cardio are "medium", and actual rest days (with zero training) would be low.
Lastly, if the cardio is just for "damage control" while prioritizing the chest, then just the treadmill work, on top of a properly setup carb cycle, should be fine. You could even tack on a few 15-20 minute walks right after the regular weight training days if you wanted. The complexes would/should be if you needed to kick the fat loss up a notch.
How long have you been doing this type training? How much weight have you gained in that time period and/or how, specifically and measurably, has your physique changed (gained X pounds, added Y inches)?