Delayed period - should I be worried?

I understand that there are a lot of factors that affect a woman's cycle, so allow me to share a few details that might help the analysis:

About a month ago, my girlfriend went on an overseas trip. She had her period about two weeks before she returned. When her period ended, she had (protected) intercourse with another man, approximately the day before she departed for her return flight. Not wanting to take any risks, she took the plan b pill.

About two or three days after she returned, she confessed what she did. We eventually worked things out, and we had unprotected intercourse (we practice the withdrawal method, which I understand to be high-risk). After intercourse, we noticed that she was discharging blood. A trip to the hospital confirmed that the cause was hormonal bleeding, likely induced by the plan b pill.

We continued to have unprotected intercourse for the rest of the month. I always withdraw prematurely (to the point that my playing safe was a problem for her a few times). During this period, I noticed a few things:

1. She's been craving a lot of food. Nothing unusual, but she usually gets this way before her period.
2. She's been sleeping one or two hours later than usual.
3. She's been a lot more amorous (which is another sign that she's about to get her period).
4. She's been complaining about the humidity (we live in a tropical country and it's the rainy season) slightly more than average.

What got us worried is that she's due to get her period this week, but hasn't yet. The last three times we had intercourse were seven days ago, the day after that, and yesterday (yesterday, we practiced protected intercourse, and I haven't been ejaculating since six days ago). We got worried enough to look at the symptoms of pregnancy. So far, this is what we've identified:

1. Her nipples aren't any darker, nor are her breasts sore (her breasts are usually sore before her period, but they aren't and haven't been since her last period).
2. There was one day that she felt sluggish (although she had a lot of pasta that day).
3. She was also quite moody one day (this was at the height of our worry).
4. She generally urinates often, and there hasn't been any noticeable increase in frequency.
5. There are no body pains to speak of.
6. There is no heightened sensitivity to smells.
7. No nausea or sickness.
8. No spotting.

She also took a pregnancy test, but the test she took was defective (no stripes).

I realize that our research isn't exactly the best way to go about it, and so I would like to consult someone with actual experience in the field of medicine. What is the most likely cause of her delayed period? Is it the plan b pill, the stress she's been under, or (heaven forbid) pregnancy?

I would really appreciate it if someone could answer my question. I understand that there is no better way than to personally consult a doctor regarding this, but my girlfriend refuses to get a proper check-up.

Thank you.