Just about anything can cause a woman to miss a month or two: stress, dieting, exercise, flying on an airplane, etc...
Another thing that's really common that a lot of people don't realize is that up to 50% of all pregnancies will end in miscarriage, many times without the woman ever knowing she was pregnant. Like if her period is late (a day, a week, a month...2 months) it's possible she was pregnant but miscarried. Bottomline: don't ever trust the woman or her claims to be on birth control. No balloon, no party!
Trust me, I'm a doctor... hahahaha!