<human>: Can you provide a brief rundown of the text in within 3 sentences?

My girlfriend and I have never had unprotected sex, and we've never had a broken condom. Naturally we thought that it would've been impossible for her to get pregnant, but then we thought about it one time and we couldn't shake the thought that she could possibly be pregnant. We have no idea why, but we can't seem to get over the idea even though we accept we're being paranoid.

When the time shortly before her period was to start, she went on vacation, which was her first plane ride. She was supposed to get her period on the day she took off (Thursday, the 16th), but she didn't get it. We both assumed that we must have miscalculated the day and we blew it off and waited for it because she said she was getting awful cramps.

But she still hasn't gotten it and we are both extremely concerned that she is pregnant, but in the situation she is in, she cannot take a pregnancy test. So I'm wondering how easily it could've been to get her pregnant in our circumstance, or if it's something else that could lead to her period being late.

There have been some things that we've been thinking about that could cause her period to be late. She has been working out recently (which she never did regularly before), which mostly included sit-ups and running, and we also considered the stress of going on her first plane ride and big vacation.
<bot>: Girlfriend hasn't had her period even though it seems impossible for her to be pregnant. What could be the cause?