I think (I am not an expert) that the white fluid is a strong indication that she is enjoying the whole sex act and is close to cumming. Believe this doesn't happen with all girls, just wth some girls.
The white stuff may have started out as some normal vaginal discharge (like some bros said above) that was still inside the vagina before the sex act and became that whitish, thin fluid after some vigorous mixing and churning by the vaginal muscles as they contracted and tensed from being close to cumming.