There is no way to know of she is pregnant this early! The nausea is either from being sick or from her freaking herself out thinking she is pregnant. After sex, it takes approx 7 to 12 days for a fertilized egg to attach to the uterus, which then could potentially cause symptoms. The earliest she could experience symptoms (if she is pregnant) is the 24th this month, but even then it's still early. I would wait to see if she misses her period before you start worrying. If she does miss her period, she can take a test with first morning urine and get an accurate result.
2 days is a lil soon to already be feeling nauseous.
It commonly takes a week before those symptoms start kicking in.
And it is very possible that she is a lil stressed out herself.. which can be making her a lil queasy.
When was the last time she had her period?