hi :)

my girlfriend and i had sex last week butt i didnt cum inside her. its our second time. her schedule of menstruation in always the first week but after i took her virginity her menstruation is delayed. the past month we had sex is that she had mestruation on the second week. My question is why is her menstruation delayed? in fact i didnt cum inside her? to me its impossible that precum can make her pregnant.