girlfriend's period a day late, is she pregnant?

(first off I apologize for this being kind of long but it's important to read it all to get the full story)
Hi,
my girlfriend just told me that her period is late. It was supposed to come Tuesday (yesterday), and has not come as of Wednesday (today).
I am 20, she is 19. We are sexually active. She is on Birth Control, and I wear a condom every time we have sex. We probably have sex 3-4 times a month because our relationship is long distance.

The only thing that scares me is that the last time we had sex, the Condom fell off and neither of us are sure if I was inside of her without it or not. I have never had sex without a condom so I do not know what the sensation is like, but neither of us as far as we can remember could feel any difference in sensation.

While changing positions, before I went inside her a 2nd time, I noticed it was off. However I'm not sure if it had just fallen off when I pulled out of her to change positions, or if it had been off prior to that when I was still going inside her.
I talked with her and she said she is feeling normal "period" stuff (cramps, stomach pains) just without the blood. The night we had sex was about a week before her period typically comes. She says she does not know if she's ever missed her period before, because she never really made note of it (I am her first sexual partner so she's just now started to take real notice in that sort of stuff).
Should I be worried? IS it too soon to try a pregnancy test? Is it too late for Plan B pill?


ALSO: She has been on birth control for years (like 5 years). originally for health reasons but now she's on it to prevent pregnancy. I also always use a Condom, and have never been inside her without one (with a possible exception as described in the original post).
I asked her "has it ever been late before?" - her response: "I'm sure it has. But I wasn't having sex then so it never really scared me like this."
I asked her, "Would you say the cramps you're getting are "normal" cramps?" - her response: "I would... Guess so. They vary a lot from month to month. I think they're normal."
Her breast are not tender/sore at all. She has not been eating much/at all the last few days (lost her college meal card and needs to get it replaced so just been living off of snacks). She also says that she has been stressed (finals coming up, really busy) and has been fatigued/tired lately (but again could be attributed to staying up late/not enough sleep for school).

Thank you