Can stressing over thinking you may be pregnant affect your period being late if all you did was try anal sex and thought some pre-ejaculate fluid got close to your vagina but are pretty sure it didn'?

yes, stressing can very much mess with your period. Once when my wife and I were dating we were having sex in the back seat of my chevelle and with out me knowing it the end of my condem had broke and i dumped a huge load in her. I told her about the rubber being broke and she stressed so bad that she was a whole month late having her period.