What do you consider "a lot" of cardio? What is a typical week of exercise?
You might be burning off muscle with your cardio and not fat. I have noticed a lot of cardio can actually shrink/destroy muscle mass but do nothing to fat (i.e. actually increasing fat %). So what you are noticing might actually have some validity as your body is now made up of more fat. If you are really curious, get a body fat test and monitor in intervals (week, month, etc.)
Perhaps try interval training & more weight training instead of "a lot" of cardio. Interval training will start to shred the fat away while not destroying muscle. After a few weeks / month of interval training get your fat tested again and I think you will be happy with the results.
Edit: Also, it would be very difficult to add bulk if you are following any of Mark's eating suggestions.