Suggest Remedy For Delay In Periods

Hi. My girlfriend and I are both virgins. I am not a teenager, I am 25 and have held out. she is 21 with the same story. This month she is already late for her period but we have not had any intercourse where we were naked before until last night. Last night I am worried that i might have slipped in the head accidentally for just a second and am also worried about the pre cum that might have been there. Please explain the chances of becoming pregnant from this. Thank you