Is the belly pooch increasing the curve in your lower or mid-back?
I would try:
1) "Static back position" where you lie on the floor and your hips and knees are bent 90 degrees and your calves are resting on a chair of suitable height.
Then do "diaphragmatic breathing". Your diaphragm muscle should drop down when you inhale and your belly will expand out (up if you are on your back). Put a sneaker on your stomach. It should go up and down.
The WRONG way to breath is for your ribcage to expand when you inhale.
2) "Progressive groin stretch" (Do a Google search). This will lengthen your mid spine and hip flexors. It is an amazing stretch but you have to lie there for 20 minutes or more for it to work.