I highly doubt she is pregnant. If she is having irregular periods she can go on the bc pill and it will regulate it for her and also give the two of you protection at the same time.
You don't even know if it went into your mouth from her's you need to relax a little!!
I dont know if saliva kills sperm, I have never heard of that, i'm not saying it's impossible. Maybe someone else can answer that for you. Irregular periods could play a huge role, if she doesn't get her period use a home pregnancy test, and ask her to go to her dr about getting on the pill so they become regular and no more worries as long as she is taking it correctly.
Good Luck to the both of you
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