What Causes Delay In Periods Inspite Of Having Safe Sex?

Hi! My girlfriend is currently pregnant, and according to her LMP, she is 8-9 weeks pregnant. Her LMP was on January 12-15, 2015, she had protected sex with another man on January 15th, and she and I had protected sex on February 12, right around the time that she said she was supposed to get her period, but it didn t arrive. Is it possible for me to still be the father?