Nauseous unprotected sex

My boyfriend and I had unprotected sex doggystyle... but he pulled out and busted on my ass. I feel extremely nauseous I just wanna vomit but nothing is coming out. I just noticed today is the exact day we did it exactly 3 weeks ago. Is it possible to be pregnant already ? I mean the only way to truly know is a test but can you start feeling sick this early on. Please let me know.