My boyfriend had put his penis for about 5 seconds in my vagina, but my period just started to begin. And I didn't not, can I get pregnant. I'm late on my period

I'm confused hun. Did your period just start to begin? Or are you late?

To answer your question anyway, the only way you could get pregnant this way is by precum. But.. I would say you are probably not pregnant if it was only in there for five seconds.

The only way to know for sure is to take an HPT. :)