Is it possible for a virgin to get pregnant in a case where pre-ejaculate fluid enters her vagina?

Being a virgin has nothing to do with it. It is almost impossible to get pregnant the day before your period as you will have ovulated 13 days earlier and the egg will be long gone. In any circumstances 99.5% of the time having a period means you are not pregnant. i.e. only about 1 in 200 women have period-like bleeding during pregnancy.