HELP PLEASE

My fiancé and I were doing it from the back and he pulled out and jacked off on my ass. But like I have anxiety about this so I made him hurry to CVS and buy Plan B. I read so many forums on here on girls getting pregnant just by letting their dudes cum on them bc the sperm kinda just dropped down there. Idk I just needed to vent and let my worries out bc this literally just happened 20 minutes ago.