Maybe this will make you feel better. Me and my gf have been having sex without condoms for like 2 yrs and she isn't on anything (birth control) i know it might sound stupid but girls can really only get pregnanat like 3 days after 2 weeks after there last peridod when they release an egg and shit. Don't be too perionoid b/c I know it only takes once but a lot depends on the time of the month or how long it has been since her last period. Don't worry it only makes things worse it and if she is worried and stressed it can actually make her period come later. Peace