If you get nausea, you should take things a little more slowly. Remember that you are carrying around a baby that's about 7 lbs and you've increased your blood by half. that's a lot of work for your heart and your lungs. You have the same rate of oxygen intake and yet so much more is needed.
Maybe this is making you a little lightheaded and you definitely throw you over to a bit of nausea.
Another thing is to eat much smaller meals. You probably want to stay away from beef because it is harder to digest by the stomach and intestines. Beans are a good source of protein. If you eat more carbohydrates, those are the easiest to digest and maybe that will help.
You have hardly any room for a stomach, much less one that has a lot of food in it. Keep drinking lots of water too.