It sounds like you had your sexual encounter around the same time that she was supposed to have her period anyway, not so?

That would reduce the possibility of falling pregnant a lot, as the ovulation (egg-release) cycle falls in the middle of two periods.

Somewhere, something is amiss. Are you sure she didn't have her period just before you guys did your thing? Or that something happened that you guys didn't notice it somehow?

Do you actually observe (by physically looking at her "down there") that she hasn't had her period, or are you taking what she is saying at face-value?

I don't want to appear like a horrible person who sees the "darker-side" of things, but, maybe there is something else going on here, not to do with her period or anything physical at all.

I know it's a horrible thought, I'm sorry, but you must consider possible dishonesty about stuff.