Hi. I Am A Nursing Mother 125lbs. 5'5" And Active.

Hi. I am a nursing mother 125lbs. 5'5" and active. My stomach for the past month or so sticks out almost like I am pregnant. It's worse right after I eat of course but it's not just belly fat, its tight when I press on it. My bowel movements have been regular. I have also been experiencing nausea lately as well. I have taken a pregnancy test and it was negative. I had one period almost 2 months ago since I had my last baby. Any thoughts?