Could be that your stomach needs to expand when you eat and at the same time uterus is also pressing onto your diaphragm which makes it uncomfortable and difficult to breathe. Try eating smaller portions instead. Like really small portions but frequent throughout the day. Don’t eat 3 big meals, you’ll have a hard time breathing. It gets better towards the end of pregnancy.
Could be Braxton Hicks cos that's how it feels. Tightening. Comes & goes. Your body is preparing for labour. Also you could be too full. At this stage your stomach feels full very quickly even with a small meal. Try to take a much smaller meal but frequent.