Anything's possible, but I'm gonna go out on a limb here and say you're safe, just based on basic physiology.
It's pretty unlikely that she'd get pregnant if it was the day after her period and she started taking her bc pills on schedule. Most pills work by preventing ovulation, which normally occurs about 2 weeks before a woman's period. It's really, really (I mean REALLY) unlikely that she would have ovulated that soon after her period, and as long as she's taking the pills, it's unlikely that she'd ovulate soon enough for pregnancy to be a possibility (sperm can hang out for like 3-5 days, so if she ovulated within that time, there might be a risk). But like I said, as long as she started her pills on time, it doesn't matter that you had sex with her on the one day that she was still taking placebo pills but not actually bleeding. Just because she wasn't taking active dose pills doesn't mean she was "off" the pill - it doesn't work like that.
Like I said, anything's possible, so be careful in the future, but I wouldn't lose sleep over this, and I'm a notorious worrywart.
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"If ten million people believe a foolish thing, it is still a foolish thing."
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