The odds of your girlfriend being pregnant are almost non existent. I really don't think you have anything to worry about.
I recommend that you never touch your penis to your girlfriends vagina without a condom. Also, you should not touch her with your hands after you've touched yourself unless you've washed your hands very well with soap and water.
It's rare to get pregnant by doing what you did but it's not impossible. Just be careful next time. Tell your girlfriend not to stress or that could delay her period. I really do think you're fine and she's not pregnant.
Wat is the day of cycle she is in?is it midcycle ?if yes emergency contraceptive pill is recommended