(( VaginaPagina ))

Not only is there very little semen, if any, in pre-ejaculate, it was on the outside of your vagina, not inside. Just one of those few would have to crawl up there. It's likely you're near ovulation, given when your period's due, but I don't say that to worry you. Precum outside the vagina has such a miniscule chance of causing pregnancy even then that I wouldn't worry.