Should I be frustrated with my gf over this?

Yes, there are birth control pills where you take a placebo for 7 days. It's so you can get a period. They've been around since at least the 70s and probably the 60s. If she takes a placebo week, then it's pretty much guaranteed that she takes a pill every single day, regardless of sexual activity and even regardless of relationship status.

There are other pharmaceutical options now, so they don't all work exactly the same way.

But there's a placebo week also because the chances of her getting pregnant during that week are very low--- and using a condom makes those chances even smaller.

I understand someone getting a little panicky, and a few days premenstrual is often a time for weird moods whether you take the pill or not.

But I am over 99% sure she's overreacting. Why? Because the chances of your setup preventing pregnancy is over 99%, unless the condom broke. And even then, the chances of her getting pregnant are likely under 3%.

As for whether you should feel frustrated, I can't tell you how to feel.

But I think it's not a bad idea for you both to better acquaint yourselves with how both methods work and with how to calculate and respond to risk.

Oh okay thanks. I thought you took the birth control pills the whole time, but the body still has the period cycle anyway. Or that is what I assumed since placebos were never mentioned before. Well I want to bring it up to her and ask if she didn't tell me during her other placebo weeks, but I don't want to come off to harsh either, or not sure how I should feel about this, as to what is fair to her, when bringing it up.

Apart from the dubious fun of causing an unnecessary fight, is there any reason to ask about earlier instances (if any)?

I guess not since after thinking about it, if it was during her period, we didn't have sex then. But she would often with me on the last day of her period since it was okay then. Not sure if she took a pill those days though.

Definitely, THE question.