Hi there and thanks so much for posting this question, believe it or not is more frequent that you think i had a client that after losing 20 lbs and looking skinny people will stop her saying how many months she was pregnant. Ok so here is the reason there is a common postural distorsion called "Lumbo Hip Complex" that mainly is exactly what you are describing. Click here for the National Academy of Sports of Medicine recommendations: http://www.nasm.org/uploadedFiles/1/CES_II/NASM_CES_LPHC.pdf
This muscles have been defined as the lumbo-pelvic-hip complex, and the thoracic and cervical spine. This muscles are where the body's center of gravity is located and where all movement begins. An efficient way to do CoreTraining is necessary for maintaining proper muscle balance throughout the entire kinetic chain.
In other words you need to do more Core training exercises, those exercises that will increase strength in the core and you will start feeling a reduction of the "belly bulging " out. Here i am proposing several exercises that you can follow. Check my blog at Medhelp to access several core exercises.
I hope this helps!