She married in May 2016. Assuming it's not a shotgun marriage, and she got pregnant in June, she would be less than 3 months into her pregnancy. It's difficult to tell if she was pregnant or just a fat slob.
The Chinese gentleman was reasonable to suspect she was pulling a fast one. She could have answered nicely by telling the gentleman that she was only 3 months pregnant and it might not have been obvious yet. She didn't have to go from being defensive to very offensive. If she thinks she is entitled to the reserved seat, surely the gentleman is also entitled to verify that she is indeed entitled.
Notice that the man wasn't even taking up the seat and he could be a good Samaritan by trying to prevent the seat from being taken by a not-so needy passenger.