Late pregnancy nausea is usually due to the big uterus compressing on your stomach and delaying your gastric emptying

Could be any number or reasons, and without getting too technical, it could be a symptom if pre-Eclampsia, could be related to reflux, could also just be your body’s response to pregnancy in the third trimester.

Hello, 

So, it is quite a long explanation, but, suffice it to say is that it is totally normal to have nausea/vomiting intermittently throughout pregnancy. As long as you can stay hydrated, you are OK and do not need to worry. 
I hope that this helps. 

Warmest Regards, 

Joseph A. Adashek, MD FACOG

Your stomach being compressed by your growing baby and also the relaxed sphincters in the lower end of your food pipe(Esophagus), both these could cause nausea and acid reflux
Take care!