What Causes Stomach Bloating During Postpartum?

I am almost 6 months postpartum with my 3rd baby. I am currently 6 pounds smaller than when I got pregnant (121 pre pregnancy, 115 post) and my stomach had gotten almost flat. It has gotten increasingly larger every day for the past week, to the point that I look about 5 months pregnant today. It also feels like a bouncy ball when I press on it and it is fairly firm to press in on it.