Condom fell off inside!

So I had sex and after my bf came, I sat on him with him in me longer. When we finally dulled him out the condom was off and was inside me! I immediately went and got Plan B pill and took it like 30 mins after. I should be getting my period this week sometime, but I'm very nervous. I have abdominal pain and nausea, could I be pregnant or is it jst nerves? Does plan b work?