Hi there. So, really, nausea with pregnancy does not start immediately. It happens because of the hormones in a pregnancy so for most women, it starts 6 to 7 weeks after conception, not when your period is due. You are likely late because you used emergency contraception/pills. Those are a huge dose of hormones which is how they work but also after that, the body deals with the hormones. They can make you irregular with your cycle for 2 to 3 months after taking them. You could take a pregnancy test now to rule it out if you want to as you can get an accurate pregnancy test 2.5 weeks after unprotected intercourse or if a condom broke. So, you can always take a test to be sure but expect that it is probably negative. Emergency contraception works pretty well if you took it right. Let us know.