Mate if you've lost a stone, whilst increasing your muscle mass, then your belly will have shrunk. Have you noticed any other parts of your body slimming down?
I remember on a previous post of yours, you were primarily concerned about losing your belly. As I think was said on that post it isn't possible to target fat loss on any particular part of your body, but as you lose body fat it will be reduced in every part of your body.
It's very common for people losing weight to develop a mild form of body dysmorphia (where they amplify negative physical aspects within their minds), I wouldn't be surprised if this were the case here Dan.
Concentrate on the massive positives that you are achieving, i.e eating properly, getting more muscular and losing weight. Keep it up!