What Causes Nausea After Taking Plan B Followed By Sex?

I had sex around 12:28-12:54 pm on 3/20 and I took Plan B one step around 6-7pm on 3/22. I was unsure if my period had started after we finished ( because I was bleeding) or if it was just him that made me bleed. I swallowed the pill but it dissolved in my mouth. I m feeling very nauseous for about 3 days now. Can I possibly be pregnant ?