What Causes A Missed Period After Anal Intercourse?

Me and my girfriend were messing around the day b4 her period!! She got naked except me I kept my boxers on for safety reazonz! We kissed and fooled around then we had anal sex for less than a minute with a condom and I didn't even cum!! Afterward she kinda sat on my lap! but or genitalz didn't rlly touch! Then I noticed I had a lil bit of precum in my boxers!! And now my girlfriend missed her period and I'm extremely nervouz!! And zhe ztrezzin!!